Hello and welcome.
Firstly there are no specific code(s). From memory I think it's mentioned in TG20 and was in BS5973 (now withdrawn) and all it says is to ensure the standards are evenly loaded!
There are 2 approaches....transfer the load from the single standard to the pair by stopping the double standard and then punching the single standard from between the pair, this ensures equal distribution from the single standard to the pair.
The second method I have seen is to stop one of the pair continuing with the single standard, this however in my opinion does NOT evenly load the double standard. The second standard supporting only 50% of the load from the lifts supported by the double standard with 100% of the load from the lifts supported by the single standard remaining in that standard....which could ofourse include the imposed load.
Other options could be to strengthen the single standard...
There is of course TRIPLE standards, gets even more complicated!
We have only recently completed a design with QUADRUPLE standards with 1,0m lifts! Very tall and very high loads!!
As for catering for the load at the base...yes they will be high. Spreaders and the base need careful consideration.