Using two pair of Xtend modems in close proximity and metal buildings- how do I improve speed and connectivity!

We are an auction company. We use a pair of Xtend modems for each auction ring we run. The rings are sometimes inside metal buildings, while the receiver is outside of the building.
When we use a second set of Xtend modems (more than one ring) we have a serious lag in data transfer and it even causes time-outs and errors.
How do we prevent this? and what can we do to compensate for the metal building? The modems in the metal building are not stationary, so we cannot put a cord from the modem to an outside antenna.
I know I am long winded- but I appreciate any suggestions!

The fist issue of two sets. Simply set one set of radios on a different HP value. Radios with Matching HP values will communicate.

Metal buildings. That is an issue. You see, RF energy does not go through metal. Your only option there is to place the radio in a window or door and run a long RS485 cable to your device.