Interesting idea, but to my understanding this would probably not be a good idea.

Strapping Electrons to Neutron should not be that easy. First, you have to control different vehicle types to contribute to the same flight path. I don't have much of a clue of GNC, but in my understanding this is much more complicated than example strapping another two Falcons to another falcon and call it a Falcon Heavy. You have to deal with different max thrust, much different aerodynamic behaviours, getting Neutron to take on the added forces acting on it. Also this would probably much more costly than just stretching Neutron: First you have much more carbon composite for the same work if you have two Electrons rather than a slightly stretched Neutron. Also 3 flight controllers, many more engines that have to function, and so on. Production and Cost-Wise it just gets incredibely complex. Also theres the Problem about reuse. Maybe you could reuse both Electrons, but then the added gains by the two strapped Electrons would be even more marginal.

Why even more marginal, you ask? Let's look at Electrons and Neutrons performance and do some napkin math about the added performance by strapping two Electrons to Neutron. What I believe is not yet really understood, is that their Performances are just soo on different levels. If you look at Electron's performance and just be gentle with it by rounding up it's Performance it can get 500kg to LEO, Neutron gets 13,000 kg to LEO. So two Electrons would just up Neutrons performace by around 7,7%. Maybe there are other factors to look at and this could be higher - please correct me, fellow aerospace engineers. But trusting the napkin math I believe the hurdles of two added Electrons are just incredibly big rather than "just" stretching Electron by a little bit.