Analyze the trade-offs. A twin power plant a/c would most likely need twice the onboard fuel capacity compared to a single power plant a/c unless the flight time is cut in half or at best considerably less. With the additional power plant and system comes the added weight to the a/c. So you have added power with the additional power plant but the trade-off is the additional weight added to the bird.

Also mentioned is the additional risk of a power plant failure, people I've known who fly full-sized planes with multiple power plants I've heard refer to this as "double trouble" reference. Logical. Additional systems comes added probability of a failure.

Cost most likely isn't too much of an influence on your decision making process so that's probably not much in way of trade-off.

I'd start with analyzing the trade-offs first and work your way forward through the decision making process.