21. Let the weight of the counterfeit coin be $a$, and the weight of the genuine coin be $b$ $(a \neq b)$. There are two piles of three coins each, and it is known that each pile contains exactly one counterfeit coin. How many times at least must a precise scale (not a balance) be used to find these two counterfeit coins?
(1) Assuming $a$ and $b$ are known, solve this problem.
(2) Assuming $a$ and $b$ are unknown, solve this problem.
21. (1) At least 3 weighings are required.

First, take one coin from each of the two piles, (1) if the weight is $2a$, then the real coins have been found; (2) if the weight is $2b$, then take one coin from the remaining two in each pile and weigh them separately, and the two fake coins can be found; (3) if the weight is $a+b$, take one out and weigh it again, then take another one from the remaining pile with the fake coin and weigh it, and the other fake coin can be found.
(2) At least 4 weighings are required.
First, take two coins from one pile and weigh them separately. (1) If the two coins weigh the same, then they are real coins. The remaining one is a fake coin, and $b$ is known. Take two more coins from the other pile and weigh them separately, and the fake coins can be found; (2) If the two coins weigh differently, one weighs $a$ and the other weighs $b$, at this point, take two coins from the other pile and weigh them together with the remaining one from this pile. If the weight is $3b$, then the two fake coins are found; if the weight is $a+2b$, take one of the two coins already taken from the other pile and weigh it again, and the two fake coins can also be found.