I think BA's systems will see the two bookings and auto-cancel one. I think it was either Smirnoff or BA97 who reported this happening before to them? I wouldn't risk it...I'd either stump up for the single return and be done with it, or potentially look at buying a return ticket ex-WHEREVER and only using the WHEREVER-UK portion (this again may work out cheaper) on ba.com

Even if you got the overlapping booking to work, you'd only get one set of TPs/miles (who knows which though)...there is an AskBA somewhere about *ahem* 'passengers of size' buying more than one seat and it said (IIRC) you only get one set of miles no matter how many seats you buy for yourself. Edit: I can't find it though, despite hunting on AskBA and the Travel Trade AskBA thing. Drat. Sorry. That's my recollection though...