No, there is no explicit bonus for completing more than one contract in the same flight. However, trying to do so can often safe you some time and money. Some test contracts are really only profitable when you do them as secondary goals on a flight which also has other purposes.
Do not hesitate to decline contracts which don't earn you enough to justify a launch and don't fit together well with other contracts you already accepted. When you decline a contract before accepting it, it will be replaced with a new one which might be easier to fulfill.
However, you should avoid declining contracts, because this will give you a reputation penalty. When you don't have any interesting contracts in the command center, wait a few days until contracts time out and are replaced with new ones. Also, the game notices which kind of contracts you accept and will then give you more of these. So you might want to accept some low-risk-low-reward contracts from your favorite category just so you can get some more lucrative ones.