The multiplier multiplies your effective average balance during the early deposit period, as if you deposited 2x as much during that period. It doesn't apply to your balance after the early deposit period.
Example
Assume:
An early deposit period of 2 weeks
You deposit 1 ETH for the full early deposit period
1 more week after the early deposit period
You deposit another 1 ETH (2 ETH total) for the 1 week after
Then:
Your effective average deposit balance, including the multiplier, would be:
(2 weeks * 1 ETH * 2x multiplier + 1 week * 2 ETH) / 3 weeks total
= 6 / 3
= 2 ETH average deposit balance
If the total of all average deposit balances (including multipliers) was 10 ETH, then your share of the total (and therefore your share of rewards) would be:
2 ETH / 10 ETH
= 20%