Stay single...
A good twin needs to be able to take-off with one engine. All the airliners are capable of continued take-off past a certain point during the ground roll. That second engine is of no use if it takes you to the tree line at the far end of the airfield and the rejected take-off puts you into the ravine.
A 6 seat twin likely needs a 250-300hp per engine to achieve this, depending on the aerodynamics and pilot technique. A 4 seat twin likely needs IO-390s. An automatic propellor feathering device would certantly help as well, although that's pretty complex to design properly.
I think you may mean that a good twin needs to be able to climb on one engine. I do not believe there is a certified twin, piston or jet, that allows take off with one engine inoperative, but your point is valid.
The early Apaches defined the often repeated joke about "flying you to the scene of the crash" as its single engine ceiling was near sea level. The B58 Baron, on the other hand, has about a 9,000' single engine ceiling. I've have had an engine failure in flight in my Baron and simply feathered the prop and continued to the nearest field to an uneventful landing, no crash involved. There's no way to know how often those events happen as it's not required to be reported, but I imagine it's a recurring event. If you're trained and proficient, it's not a big deal. If you're not prepared, it is absolutely unforgiving and deadly.
The 6 seat aircraft you've spec'd is a solid Baron and the 4 seat would be a Piper Commanche. Both good, stable twins. The issue with them is that they're not experimental nor inexpensive to own and operate (I owned a B58 Baron for many years and still have the mental scars of parts and labor costs {side note, I was weighing the cost of a custom made SkyDynamics sump and intake and then remembered it cost almost $7,000 for a new starter adapter for an IO-550 on the Baron [that's not the starter, just the Rube Goldberg gearbox that the starter mounts on], and suddenly, that cold air intake with sump seemed inexpensive. Yeah, I bought it}). And although fuel isn't actually twice the cost, it's a lot. I owned a Bonanza and a Baron and I can tell quite accurately that a B58 fully loaded is about 36% more expensive in fuel than a fully loaded A36 Bonanza when calculated block to block. Those two airplanes share a common airframe design and are about as good an example of what you could expect by comparison if the RV-10 became a twin, although a comparison of the Velocity versus Velocity V-Twin is another good one and experimental too; I just don't have hands-on experience with both like I do the Beech's to provide meaningful data.
The thought of a twin RV-10 is interesting though. If you're thinking to go faster, or just thinking twin engine to have the extra engine in terms of flying over the mountains, at night, in IMC...the trifecta of instrument aviation - you should consider the Velocity V-Twin. It's about $250K+ to build and a lot more hours (and I hope you enjoy composite work...), but it is very nice airplane. I came very close to buying a kit a couple years back, but then decided on the 10 after meeting with a couple V-Twin builders and coming to the conclusion that it will take closer to 4,000 hours to build (and I'm hoping to be flying it with a little life left to enjoy it). It would be fun to have though, if you can get past the construction time and cost.
Second side note: I laugh when I read folks talking about cost cutting on their airplane builds, saving $10 here and $50 bucks there - all the time I'm thinking that buying
"expensive parts" for an experimental airplane compared to parts from Beechcraft is like comparing getting hit with a fly swatter versus a sledge hammer. Aside from the idea that penny pinching on an airplane that may have cost you north of $150K to build is ironic, the cost of a twin is an upward parabolic curve. The starter adapter example I gave is one thing, but consider the cost of buying and building an elevator for your RV-10. Now consider that a new elevator for a B58 Baron, in 2017 mind you, was $23,000 PER SIDE. I'm not sure that it's $1000 for all the parts to build a complete RV-10 elevator, but it's in that ball park - a much smaller park to play in.
I say all that to make the point that thinking about twins is fun, AND, I thoroughly loved flying my Baron, BUT, the cost of owning and operating a twin - even an experimental twin, is MUCH higher than the wonderful RV-10. Even more so, the importance of recurrent training One Engine Inoperative and being highly proficient on your airplane just about mirrors the O&M cost. Have fun, just have the right expectations, and bring your checkbook.