here we are taking as hex
b+d =bd
where b is one point and d is 2nd point, result should be bd point in concatenation
for your easy dec result will be 189 not 1113
I guess you do understand that this concatenation you're doing is
not point addition but rather a different operation (which makes me feel better), so now you're looking for math properties of this concatenation thing?
Well for starters the operation you're doing is equal to a*len(b)*4 + b [there are 4 bits in a hex char] in terms of the real addition so so you should be able to use it almost anywhere you need an
Ax + B style expression.
this is simple math operation to calc, a*len(b)*16 + b
but based on x,y points, there will be 2 snaorio, if concatenate under mod p, will be calculated, and result will apear on curve other not at curve, that will be apear based on point formula,
and if operation everything will be on curve always