what is the problem

http://everycircuit.com/circuit/5236813872758784

consider insert two small internal resistances in the batteries

It's because you attached the ground to positive.

detach the ground and attach it to negative of one of the batteries and add two one milli ohm resistances to each of the batteries in series.

You shorted the voltage sources

It seems to me that the EC program developers have overlooked a simple problem....we find this similar problem when putting two switches in parallel too...I suspect that they have given switches and supply sources a zero resistance so that current/voltage circuit readings are not effected. That is fine until we come to calculating the total resistance of two in parallel where zero occurs in the basic reciprocal calculation. There is probably an easy way to account for this with a slight programming alteration (eg with an 'if this then that' condition) so that parallel switches and sources act properly as they would in reality.....Let's hope the program developers get to grips with this soon.

I don't mind it. Gives me more options to fine tune the Q factor of LC circuit for instance.

Voltage sources need series resistors, specially if you put them in parallel. So they can share current.

Connect diod at the outputs of each source.

You do a Short circuit. Remove the connection between plus from the batteries to gnd.

A few observers here have noticed the obvious which I overlooked...the supply positives are directly connected to their negatives ...ie at the earth point. Stupid oversight of mine...well done for pointing it out guys...The basic problem which I latched on to without that, still exists though....The EC developers seem to have attributed zero resistance to the supplies and so created the problem of adding parallel resistors with a zero incorporated....which locks out their program....that is called a 'bug' in computing (not to be confused with a spy/virus type of bug).

I overlooked that as well. Anyway EC is an ideal environment, so very few passive components have an internal resistance here.

Yes, it's just picking flies really...it gives us a little chatter break while we all enjoy the fun together with this great EC program.