Having sexual relations with a female and her friend is always acceptable as long as the total score of said bitches hotness equals 10 or more.
The 3some plus rule
While having sex with a 9, one only needs to include a 2 or greater to make said intercourse awesome and braggable.
Coversely, having sex with a 5 and a 4 is just average and the particapator should be ridiculed for banging 2 Gremlin
In extreme situations the hotness scale can be ignored for certain features such as huge tits or no gag reflex. Of course these attributes must be reviewed by a panel of your peers.