Important note: You can't connect both amps to that cab without blowing something up. You need two cabinets.
There are some cabinets that support bi-amping, which is using two amps with one cabinet. Your cabinet does not. Your cabinet has two 1/4" inputs wired together in parallel so you can daisy-chain additional cabinets.
Yes you can drive both power amps from one preamp by connecting the preamp out from one head to the power amp input on the other head. When you do this, the head that you take the preamp out from is the one you should plug your bass into. The gain and EQ controls on that head will be the ones that control the sound, the preamp and its controls on the other head will be bypassed.
Then you can plug the speaker outputs of each head to separate speaker cabinets.
Now let's talk about making your rig louder. In order to make your rig sound twice as loud by buying more amps, you have to increase your total wattage by ten times. That means if you have a 100 Watt head now, you'll have to have a total of 1000 Watts to double the loudness of your current head. Plus you'll have to have speakers that can handle that load.
A much better way to increase loudness is to get a better speaker cabinet that is more efficient and reproduces more high frequencies. Yes, I wrote higher frequencies. If you are pumping lots of Watts but still can't hear your bass, then the problem is more likely EQ than power. Make sure you're not cutting your highs and mids too much and just boosting lows. High mids are very important for making the bass heard with the rest of the band. You mainly want the lows to go with the kick drum.
Try turning that high frequency control on the back of your cabinet all the way up
After looking through all your links and gear, I recommend using the B5R only, buy and use a Speakon cable (because 1/4" speaker cables are bad and dangerous at high levels - dangerous to gear, not people) to connect the B5R to your cabinet, crank that high frequency control on the back of the cabinet all the way up, and work with the Ultramid control on the B5R to make yourself heard. Yes, you'll "only" be pushing 300 Watts RMS into 4 Ohms, but you'll have a good amount of headroom which is worth solid gold for bass and PA amplification.