Two groups of Republicans in two different ballrooms will be waiting for President Reagan when he makes his first campaign appearance of the year at the Conrad Hilton Hotel tonight in Chicago.

One may be disappointed.

About 2,000 party regulars who have paid up to $2,000 apiece at a fund-raiser for Sen. Charles H. Percy (R-Ill.), chairman of the Foreign Relations Committee, who is up for reelection next year, are virtually certain to hear from Reagan, the featured guest at the event.

However, 300 supporters of conservative Rep. Tom Corcoran (R-Ill.), who is expected to challenge Percy for the GOP Senate nomination, are not so certain to see the president as they dine one floor above. Many are conservative activists to whom Percy is anathema.

Even though he has long made it a practice to stay out of Republican primary contests, White House officials say the president has "no plans" to attend Corcoran's rival fund-raiser. But he could decide to go at the last minute.

The twin political events pose a delicate problem for the president. The official White House explanation for Reagan's appearance is that he is "fulfilling an obligation" he made during the 1982 campaign and that Corcoran is an "undeclared" candidate, spokesman Larry Speakes said. He didn't mention, however, that Percy is also an officially "undeclared" candidate.

Speakes said Percy has supported Reagan loyally in the Senate over the last two years. "The president has always supported incumbent Republican senators when they have supported him," Speakes said.

Except for the president's attendance at tonight's fund-raiser, Corcoran said White House chief of staff James A. Baker III promised that Reagan would remain "neutral" in the primary. "We've already accomplished our objective," Corcoran said.

Reagan's practice of not interfering in Republican primaries stems from the so-called Republican 11th Commandment, a rule he has often preached, which goes: "Thou shalt speak no ill of another Republican."