Like diydave said cut the existing drive shaft after the universal and use a matching spline to fit the front of the second trans then install a universal joint. Or find, fit, or weld a yoke on the front of the second trans and connect with a universal joint, this will also allow for a small amount of misalignment between trans 1 and trans 2. My dad chose the later on a home built tractor many years ago, and it never broke. Look at an older four wheel drive with a separate transfer case this will give you a good idea of how itís done.
You never stated if truck is 2 or 4 wheel, keep in mind that if you add a trans to a 4 wheel drive you will be moving the transfer case back reducing the rear drive shaft and stretching the front one. You could also change axels to lower ratios thereby reducing speed and increasing torque.