Combination of acceleration signals
Acceleration is a 3-vector, so yes, a plausible 50-50 estimate is
$$ \mathbf{a} = 0.5 \mathbf{a}_1 + 0.5 \mathbf{a}_2 = \sum_{i} w_i\mathbf{a}_i$$
If you have a reason to believe more in one measurement than the other, you can use different weights, as long as they sum up to 1:
$$ \sum w_i = 1 $$
Of course, make sure that these vectors live in the same coordinate reference frame. If there is a rotational offset, then you have to convert it from one to the other.