Question

why we are not allowed to connect ideal voltage sources in parallel

Answer 1

I don't really know for sure, but maybe it's because ideal voltage sources has no resistance and when you connect them in parallel, infinite amount of current will flow (short circuit).

Answer 2

as far as i know..when 2 elements are connected in a parallel connection the voltage across them must be same .So when 2 voltage sources are connected they must have same potential difference across them which is not possible unless they are of same value and in the other case(different value sources) it is same as the greater valued voltage source since it overtakes the other one .hence there is no use of connecting both of them in parallel

Answer 3

1) Ideal voltage sources are not available in the real world, only in theory

2) Remember that an Ideal voltage source has zero internal resistance; it can maintain the ideal voltage (or 'potential difference') independent of how much or how little load it's connected to; and finally it can supply (or absorb) any amount of current.
Now connect a load resistance to the Ideal voltage source. As the load resistance is decreased to zero, the current supplied by the ideal voltage source approaches infinity.
Here's the problem: if you connect a 2nd ideal voltage source, you're basically saying (in your model) that you have infinite power coming from your Ideal voltage source. This is why it's not allowed. It's quite similar to the "division by zero" paradox.

Thankfully you can't do that in the real world!