New Delhi — India and Pakistan have been called the ''terrible twins'' of the subcontinent. One is the world's most populous democracy; the other has been, for most of its independent history, under military rule. Within their quivers, they hold some of the world's most deadly and sophisticated arms. Their two leaders, Indira Gandhi and Gen. Muhammad Zia ul-Haq, distrust each other. And, as Vice-President George Bush heard on his trip to both countries this week, United States relations with one cannot be divorced from US relations with the other.
There are multiple irritants in Washington's relations with both New Delhi and Islamabad.
Perhaps more than anything, Afghanistan provided a central fulcrum for Mr. Bush's talks. He spoke of the ''brutal Soviet invasion'' - the first time that diplomats can remember any foreign leader speaking so forcefully about the subject on Indian soil. For, although India has strong objection to the Soviet presence, it has never publicly condemned the Red Army's occupation force.
The vice-president is said to have requested Mrs. Gandhi to use her diplomatic skills with the Soviet leaders on a possible negotiated withdrawal. Indian sources, however, perhaps reflecting the skepticism of the Indian premier , have noted the hardening of the Soviet position since Konstantin Chernenko assumed control.
Mrs. Gandhi was very interested to hear of President Reagan's trip to China and the nuclear agreement it spawned. India is having problems with the US over the supply of nuclear spare parts for its reactor at Tarapur, as it has not signed the nonproliferation treaty. China hasn't either, Indian sources are quick to point out.
Yet, underlining the talks was the US special relationship with Pakistan, and specifically, Washington's $3.2 billion program of military and economic aid.
Mr. Bush arrived at a time when relations between the world's two largest democracies are rather cool. Mrs. Gandhi told a recent interviewer that, in America's global strategy, no place for India has been found. And, on the other side of the coin, Pakistan receives preferential treatment in Indian eyes.
And despite Mr. Bush's efforts to explain why Washington attaches such strategic importance to ''front-line'' Pakistan, a ''buffer'' against the Soviet Union in Afghanistan, the Indians were not at all convinced. Why then, Mrs. Gandhi is said to have asked, are 11 out of 16 divisions of the Pakistani Army facing the eastern border with India, well away from the Afghanistan frontier?
General Zia is likely to counter that, with India's overwhelming military superiority, he has no choice. Three wars have been fought between the subcontinent's ''terrible twins.'' Each time, India has been victorious in a continuing hostility now spanning 37 years.
Beneath the surface of the talks between Gandhi and Bush was bound to be the sensitivity that Indians have long felt: that the Western world, particularly the US, often neglects and ignores them.
The US is now India's main trading partner - imports and exports totaled $8 billion last year - and Indians consequently prickle at Washington's tendency to view them as being ''in the Soviet camp.'' They are annoyed that American foreign policy has no place in its geopolitical strategy for nations nonaligned. And they smart that the world's most powerful democracy often seems most comfortable with military regimes - in this case, Zia's Pakistani regime.
The Pakistanis feel encircled by hostile neighbors, and their military regime is said to be increasingly irked by US congressional amendments and congressional staff reports that would restrict US military assistance, tying it to human rights or to Zia's assurances that Pakistan is not developing a nuclear device.
How strongly Mr. Bush embraces General Zia at a time of growing Pakistani discontent with his military regime will say as much to Mrs. Gandhi about the future of Indo-US relations as her many hours of talks.
With a distinct hardening in Kremlin attitudes on the Afghanistan war, General Zia reportedly will attempt to convince Mr. Bush that Pakistan needs even more arms. At the top of the general's shopping list are reportedly two E-2 C Hawkeye planes, with their sophisticated early warning systems, plus air-to-air missiles for Pakistan's 40 F-16s - six of which have now arrived, via Saudi Arabia, from the US.
Mr. Bush said in New Delhi that ''we have made it very, very clear to the Pakistanis that it is not in our interest to upgrade the weapons they already have.
Growing strains have already developed between the US and Pakistani administrations on US covert assistance to Afghan resistance groups.
Pakistan's 3 million Afghan refugees have become a destabilizing political and economic force. This, combined with growing anti-Americanism among political party leaders and the country's intellectual elite, would work against a more visible US presence.
Skeptics among the foreign diplomatic community suspect that no new ground will be broken during Mr. Bush's trip. It was a noble effort to brighten the administration's foreign policy look. Fortuitously, both leaders of the subcontinent are interested in cosmetics themselves, and both are using Mr. Bush's visit to give their own images a fluff.
Mrs. Gandhi faces possibly her most formidible election later this year. And General Zia, who declared that elections were imminent when seizing power nearly seven years ago, is now deciding how to secure his own position within a controlled democratic framework.