You're talking about a '88 700, right? I'm talking about a 240 car's engines, but they ought to be the same -- every 240 that I've owned (since '78), at least as long as I can remember, have had a pair of alternator-water pump belts, including B21, B23, and B230 variants. [Well, honestly, my memory may not be reliable about the B21's, as it's been a long time, but I'm still using the B23 as well as two B230's].
The crank pulley has three grooves, two for the aforementioned alternator-water pump; and one for the A/C compressor. The A/C, in turn, has two grooves as well, with the second groove driving a single A/C to P/S belt.

The only reason why you might possibly be different is the turbo variation (mine have all been n/a), but there's no possible rationale for giving your turbo motor only one belt!

BTW, the reason for two belts is, besides redundancy, that because the belt also covers the waterpump, the arc of contact of the belt around the alternator's pulley is diminished -- less length of contact promotes more slippage, so doubling the belts ensures that the alternator is less apt to slip under a heavy load.