groups don’t matter, the principle is the same as with matrix multiplication:
with concatenated inputs you have
batch x 11 @ 11 x 10 = batch x 10
equivalent with split matrices:
batch x 10 @ 10 x 10 + batch x 1 @ 1 x 10
Now, two separate matmul/conv ops with smaller tensors are likely to be slower than one with concatenated inputs. The only upside of avoiding cat is memory savings (also note that inplace summation is possible).