If the two parties merged and formed a united socialist front, their combined vote strength might be enough to gain a significant electoral victory. In such a case, they might start nationalising industries, like Britain post-WW2 did.
In that case, they might be able to gradually, legally, develop a socialist state in France.
Now, a communist state? Maybe. The mechanics are there, with the French system of communes and small administrative areas. The only toughie is abolishing private property totally, which might not go over well with the far-right military folk. We might see another full-on civil war if they tried it too quickly. Assuming that, by "communist", you mean real communism, not that Stalinist/Maoist bullcrap.