6kw ~ 8hp, so, that's around 16hp, so you are giving up some power. On the positive side, splitting it across two props is a small improvement but for most purposes, HP=HP.
In most conditions, that's plenty but in some conditions, it will be a bit light. We had a 25hp on our 34ft
Gemini and a few times we needed all of it...such as motoring up a narrow channel directly into a 30kt
wind doing around 2kt at full throttle. A fair number of folks run with 15hp outboards with Geminis. As long you accept the reduced power and capability, it should be fine.
Be careful of claims of "equivalent HP" most come from the automotive world. The reason a 100hp
electric motor can match a 200hp ICE in a car is because, the
electric motor has max torque from 0
RPM. An ICE typically has to build up
RPM to reach max torque. HP = Torque * RPM, so the low end torque means at low RPM, the
electric motor is producing more HP. In reality, your average ICE never puts out it's rated max HP in real world use. Cruising down the freeway a small car might be putting out 50-60hp
electric or ICE.
With
displacement cruising boats, acceleration isn't a big consideration. Plus since the prop can slip in the
water, the ICE can quickly build RPM and get up into peak torque. As a result, HP to maintain cruising speed is the limiting criteria and at steady speeds with motors appropriately sized, electric or ICE are running at or near peak torque and HP=HP. Reality is you almost never see people using full throttle while
docking...unless they don't know what they are doing or things are long since out of control.
If you can feed them, electric is a great option. The problem is storing the feed for electric motors is much more difficult if you want any significant range.