Hopefully someone can help me out with this one

Looking for a flight out of LHR to SIN in business - for the whole trip the TP calculator tells me 0 avios and 160 TP.

Since I assumed its actually 2 LH flight, I thought that it should yield more TP. So if I break down the calculation into 2 flights LHR-DOH and DOH-SIN it shows me 4000+ avios and 140TP for each of the flights.

I assumed that with a return flight I could get 560 TP - is my assumption correct, or do I need to get 2 separate bookings for that LHR-DOH-LHR and DOH-SIN-DOH?

Thanks