How about using both lights?

depends on how big of an area you need to cover. the good thing is you can start with the 1 light, and save electricity as you slowly increase area of coverage with hanging distance and power knob. Focusing only on the existing canopy until you need a 2nd light -- may need to dim them both to start and continue a similar ramp up as before with 1 light. DLI -- 35-40 DLI as calculated from diode should be good starting ballpark for normal hanging distances at ~full pwoer. (photoperiods you typically shoot for 66% of total power of lights on an 18/6 schedule, then when you ramp up to 100% on 12/12 you maintain same DLI) Daily light integral -- wiki or any reputable site wil have good info as this is not new knowledge. It's been around a long time. Some plants need more or less, but th emeasurement itself allows for an apples to apples comparison of light provided regardless of hours of operation or size of garden. Understand the gist of it and you can avoid a ton of misguided information in regard to light.

Go by DLI and amount of light produced + plant needs. They may not need two lights depending on the tent size. or they may not need much more and when using two they might be both dimmed down to 40% vs 100% of one. Experment aorund with one or the other and try and find some PPFD maps for the lights on their own. From their you can calculate DLI fairly easily.

You could run both together(though being different brands/wattage the footprints and optimal hang heights could differ) or you could use the 100w in a space dedicated for veg and the 150w in a space dedicated for flower- so you can run perpetual harvests and keep the buds rolling faster.

I don't know how small your tent is, but I would install both (and supplement lights) and used all of them during flowering stage.

I would just run the FC-E1500 from start to finish, switching lamp is annoying ...