Dear Fahad
I think it is not an applicable case of planning, as each company is working for its benefits and losing one of its channels to the other company is not an economical issue.
But any way if we assumed that can happen, all what you need is to configure your BTS with the new frequency that is used by the other company. As the BTS could supply all the frequencies of the 900 MHz band, but each company can use only a part of the band.
Problems you will face is depending on the planning of the other company. You have one of two options:
1) Use a frequency that is used by the other company in the same cell.
or
2) Use a frequency that is NOT used by the other company in the same cell.
in the first case you have to be sure that the other company will not use this freq during your calls, and then you will not face any problems.
Hope it got the point.
Thanks