Gates separate, driven by unipolar zero cross detection circuits comes to mind.
Would need to detect sign of slope to figure out which SCR to fire, and which
one to turn off. Note SCR turn off requires -
A variation on bertus's suggestion would be to use an opto triac (Such as an MOC3052M) in place of the small triac. That would give you isolated drive. You could do with giving more context information about the circuit that you plan to use to drive the gates of the SCRs. One common way to do it is to use a pulse transformer with two secondary windings. (One for each SCR.