If algorithm A takes an input of size n, and has a time complexity of O(a^n) and a space complexity of O(1)
First of all we do not know any exponential or sub-exponential time algorithm that requires only $O(1)$ space, having said this, it is difficult to reason about a hypothetical algorithm "A" because the spatial and temporal complexity are closely linked to the functioning of the algorithms and are (generally) proportional to each other, however, the answer to your question is no.
Let's try to reason starting from a $3SAT$ instance. Now we know that $3SAT$ is NP-Complete (best known time complexity for $3SAT$ is currently $O(k^n)$ with $ K=1.439$ for a deterministic algorithm) and $3SAT$ $∈$ PSPACE , in fact space complexity of $3SAT$ is $O(n)$. Now it is difficult to imagine how increasing the space to a constant $k$ (in your question $k = 2$) may lead to a decrease in the execution time of the algorithm that solves $3SAT$ ... in fact always keep in mind that an increase in space also corresponds to a proportional increase over time.
Let me conclude by recalling the relationships between temporal and spatial complexity classes for which we believe all inclusions to be strict: