Use the JGSME Mod Enabler (link in sig) - when you want to play SPQR activate the mod through JGSME and launch from the rome.exe as normal - when you want to play BI deactivate SPQR through JGSME and launch from the bi.exe as normal. You need the vanilla version of BI for this to work (so basically, straight out of the box - or with the official 1.5/1.6 patch).