just to add some info
with your current situation, you can already simulate how good will be your 1800 coverage after the extra-loss.
Do a drive test, located in the 1800 zone.
Remove 5dB from the results.
Are you happy with this reduced coverage ? Can you still cover the 1st village ? Can you also cover part of the 2nd village ?
If yes : go on, add 2 1800MHz trx.
Now, does the second village connect on the 900 TRX or on the 1800 TRX ?
If 900 TRX : can you ensure that only 2 TRX for this village will be enough ?
If yes : downgrade the 900 zone to 2 trx only.
Also, yes you are right : when MS is doing the call setup, it is located on the 900 TRX. The BSC cannot know what will be the exact rxlev in the 1800 trx. Atto explained it well 🙂