Portable Generator Temporary Connection
Heck No.  Checklists of switches to throw in sequence are absolutely unacceptable. I know this is "new to you", but this is well-hashed territory to safety investigators and most people familiar with electrical.  It falls in the "facepalm, what were you thinking" category.

This may seem counterintuitive, but for a temporary fix, you permanently modify the wire.  You de-energize the entire system, then run around with flashlights physically severing the mains connections, and physically splicing in the generator connection. Like you'll never use it again, but leaving it intact for potential future use.   

For instance I would physically remove the main breaker from the main panel and replace it with a (listed) blanking cover, and/or remove and insulation-wrap the mains feeds.  Then hard-splice the wires to the garage so they go to the generator instead of the garage panel. 

Walk that whole thing, double/triple check it, have the inspector visit if that's a requirement, then light 'er up.  That's what I do for temporary work.  



For permanent work, obviously, I'd put in a proper interlock.  This is not as big a deal as it sounds.  I would start by installing a small** sub-panel right next to the main panel.  This would have two "main breakers" in positions 1-4, with an interlock between them (rather simple affair).  One would feed from the main panel, the other from the genny.   Then I would decide which circuits I would ever want to light from the generator, and move those circuits to the subpanel.    The original main panel stays mains-only. 

 
src

I gather you want to use the wiring to the garage subpanel in a bidirectional manner, feeding the garage from mains, or feeding the house from genny.  This has come up on this forum before, and there's just no way to do that even remotely safely.  Bite the bullet and lay parallel cable. 



** by my definition, "small" is 42-space.  Panel space is dirt cheap and often comes with free bonus breakers, so a net win.  Whereas, running out of panel space is a catastrophe.