actually i think i remember someone did a model on this, dont remember what the result was. However, the loss of the a hydrophobic tail equals loss of hydrophobicity, the overall size/volume might get smaller. Now will there still be a cell, hmmm dont remember.

Each tail is long, and some have cis double bonds which make them stick out at weird angles, making packing the tail side by side into a rigid structure not possible. With two of them held together by the glyceride molecule, it does the opposite in that it gives the cell membrane more rigidity than just a single tail would, plus the single phosphate on the top would have more room and not repel the other phosphates from the neighboring ones.

the question is, whether you mean to have
only one FA and one phosphate on glycerol
one FA and two phosphates on glycerol
one FA and one phosphate on two-carbon substance

the two tails create a unique shape of each molecule. If you only had one tail you could probably still form vesicles, but much smaller than those you are able to form with two tails, because a one-tail phospholipid would have much more of a conical shape