There are no separate time fields to log single/multi pilot time. This data is taken straight from your aircraft properties on the Tables - Aircraft page !
e.g. you log 30 minutes Total Time on a Cessna 172, then PILOTLOG automatically considers this as Single Pilot time.
Some commuter companies schedule a two-men flight crew on a single-pilot rated aircraft. You can then simply modify the properties for that particular aircraft. However, changing the properties will affect all flights ever done with that aircraft registration.
Should you want to log some flights as single pilot time and some flights as multi pilot crew on the same aircraft registration, then continue as follows :
On the Tables - Aircraft page, create a new aircraft with a the same registration. For instance, if D-KLWA and N340PV are single pilot aircraft, you can add a new aircraft with tail number D-KLWA(MP) or N340PV+ as multi pilot aircraft in your database. The trailing + and brackets are filtered out when printing your official Logbook.
For general information about flight time logging, continue reading here :