It looks like you’re going to perform a manual frequency planning, right ? That’s why you’re trying to find a way to split your frequency band in two parts : BCCH & TCH.
If you choose to alternate BCCH and TCH, as you said, the main problem I can see is that you have to be extra-careful : it might be easy to make a mistake and plan adjacent BCCH/TCH in the same site, or even in the same cell.
Otherwise, if you are extra-careful (and i’m sure you are), this is an interesting approach.
Quite original, but that might actually work, if you can ensure that your C/I stays above -6dB (limit of adjacent interference impacting the radio quality). In other words, you need a good cell planning with sharp borders.
But additionally (that’s what everybody is talking about in this post), it is surprising to see as many BCCH’s as TCH’s frequencies. Unless you have only 2 TRX per cell, this can’t work : your BCCH quality is going to be too good, and your TCH quality too bad.
As a rule of thumb, if you plan 1 trx BCCH + 3 trx TCH per cell, you should have 2.5 to 3 times more frequencies for TCH than for BCCH (approx.). For “easy” manual planning, it should be 3 times more.
I hope it helps… regards,