In collocated cells (one 900 and one 1800), of course, you’ll have one BCCH in the GSM900 and the other BCCH in the DCS1800.
It is a choice you’ll have to make when you design a site : do you want collocated cells or multiband cells ? (= dual BCCH cells or single BCCH cells ?)
The multiband cell solution contains some drawbacks, depending on the vendor (it might not be possible to have GPRS in the inner zone for instance).
But it also brings nice advantages : simple parameters, only one cell to manage, less neighbours, intracell HOs…
You can’t design a multiband network if you have no knowledge about all that… improvisation is not a possibility 🙂 with all due respect, if your boss is asking you to do that, you should really go to some trainings or ask documentation from your vendor.