For everyone finding it difficukt to find time to write, I'd just say let something else go...whispers...housework.
I wrote book one when I was working full time and had babay twins. I don't say this to show I'm superwoman, I'm so not. I say it to show you can fit it in, but only if you give writing time priority. Cut out something else - gym,coffee with friends, ironing,MNing .
Ponymum - I would be tempted not to give up on book one. f you gt a detailed feedback from your first sub, then it has something. Rejections are usually standard three liners.
However, most agents do want the first three chapters My agent says he can tell within a couple of pages if he likes it!!! So you are going to have to re-work them to be the very best that they can be. They need to draw the reader in, not necessarily with explosions and murders, but with something the reader feels they need to know more of.
They must also set up your MC, setting and genre.
Don't worry that you feel the first book isn't as good as the second - you're learning on the job, so your second will be better. I cringe at the thought of book one, but it was the very best that I could do at te time.
Worse to debut with something you can never match, I tell myself. LOL.
That said, if you really cannot face a rewrite then don't. Nothing is ever wasted. Treat book one as a steep learning curve.
As for multiple subs, well agents don't like it. Tough titties, I say. Sub out in batches of five. Don't claim it to be a sole sub, but don't announce it's part of a dump sub either.You have the right to reamin silent etc...
If you do sub book one, definitely mention book two. No agent wants a one hit wonder, and the idea that you are prolific is what pleases. I wouldn't though, send the chapters of book two. These may change radicaly by the end of the novel, you could edit them out altogether. Instead, I would add a final para in your cover letter saying what you're working on, the title, how it links to book one, how far you've got etc.
Mention if it's a series, of course.
Now, I'm no expert,would never claim to be, so take or leave any of my advice.