So I would be allowed to keep placing marbles on the two balances until they both said 49g?
Once I have done that I have 100g. The actual weights would be 48g and 52g.
Using your sample weights:
-3 +1
5.4 5.9
5.6 6.1
6.3 6.3
7.6 6.8
8.2 7.1
9.2 7.5
9.7 8.3
Say I pick the (-3) balance first. Placing the first set of marbles will make is say 49g. I then place the rest on the second balance for another 49g.