Ideally you'd want a minimum of one quarter wavelength between the antennas, more if you can do it.
Issue is two fold:
1. Having an antenna too close to a transmitting antenna can throw the radiation pattern off, making the TX antenna slightly directional.
2. Depending on the TX power, if the two antennas are too close to each other, the TX energy can couple to the scanner antenna and do two things:
a. Overload the front end of the scanner causing to to lose reception when the other radio is transmitting.
b. If the TX power is high enough, you can damage the front end of the scanner receiver by overloading it with too much RF.
Personally, if it was me, I put the MFB-1503 at the top of the tower where it can do the most good. I'd avoid putting the scanner antenna next to it to prevent any issues. Instead, I'd mount the scanner antenna farther down the side of the tower. A receive antenna spaced off the side of the tower is going to see very little impact from the tower, compared to the reflections a transmitting antenna would see.
Doing that would minimize any overloading of the scanner front end, reduce the chances of damage, and still work very well.