We cannot use horizontal micro programming here as its degree of parallelism is one or more than one i.e. it cannot guarantee 2 control signals to be active at a time.
But we can use vertical micro programming with two groups. Given that micro-operation field size is 9 bits, division can be done in one among the following ways :
(1,8), (2,7), (3,6), (4,5)
Also it is known that 46 micro operations are supported, so we need to select the most efficient group considering very small wastage of space
With (1,8), number of control signals possible = $2^{1} + 2^{8} = 2 + 256 = 258$ (which is very large than 46)
Similarly with (2,7) we have = $2^{2} + 2^{7} = 4 + 128 = 132$ $> 46$
With (3,6) we have = $2^{3} + 2^{6} = 8 + 64 = 72$ control signals
And with (4,5) we have = $2^{4} + 2^{5} = 16 + 32 = 48$ control signals which is very close to 46.
So we need two vertical micro programming fields with 4 and 5 bits each to support 46 micro operations efficiently.
However the question would have been more apt if it was "with a parallelism of atmost 2" as vertical micro programming works on the principle of none or one.