Rejoining Previous employer and stamping H1b

[stampwithgrace]

I have a rather complex situation here and will try to explain the best I can. Looking for the best solution. 

I used to work for company A and worked for 5 years. I have had two H1b stamps with them and had no issues. My current Stamp which expired last month is with Company A. 

But I resigned and joined company B in Nov last year. Now I am joining company A again. 

I am currently outside USA for a business trip and need to get a stamp to return back as a Company B employee after which I will resign Company B and join company A. 

My Company A's new I 797 is approved but will not be active till Jan 2018. 

My question is can I go for a dropbox with a Company A I 797 and Petition even though its for future employment but enter USA on the I 797 of COmpany B as I will not be joining company A ? Company B I 797 is valid for 2 years. So advantage of Compay A is that as its a fresh I 797 ill get a longer stamp. 

Or should do the right thing and go for a Dropbox with company B. 

I feel safe with company A as I have had two visas with them. 

Company B is big and reputed and I am eligible for a Dropbox but just feel safe with A. 

 

I hope my scenario is clear, if not I can try to describe better.