Wonder if anybody has been able to "split" their mileage accrual between two airlines on the **same** journey.

I am currently a PLT on AA and Silver on BA. Need 90 tier points (before July) to requalify for Silver.

Am soon flying Biz on AA metal on AA flight numbers (MIA-LHR-STR-LHR-MIA) on a ***single*** ticket. This will give 320 TPs at BA--of which I ONLY need 90 to requalify. Checked with BA, and was told I would eventually lose the other 230 (as I will not have enough for Gold=1500 TPs).
I don't want to do that.

Would it be possible to have MIA-LHR and LHR-MIA mileage credited to AA and LHR-STR-LHR mileage (80 TPs in all) mileage credited to BA? I can make up the other required 10 TPs in a later trip. Likewise, I can use the MIA-LHR-MIA mileage to requalify for AA.

Alternatively, I could request outbound mileage on one airline, and inbound mileage on another. Has anyone been able to do this? Thanks.