employment overlap and affect on I-140

[savisavi]

I have I-140 EB3 approved from previous company. I had applied I-140 EB2 with current company.
I am leaving current company in 4 weeks but I just heard my PERM approved and I-140 will be filed.
If I join the 3rd company and if I need to extend my last day in current company, there will be overlap of couple of weeks just in order to get I-140 filed, ported and interlink etc.
Is it illegal to have employment overlap ? any impact on green card ?