The advice given so far is terrific. I understand what you say though about no double shotguns sxs around for you to borrow to "practice". I bought an old Stevens double barrel 12 gauge side by side on auctionarms for $180 so that I could practice. I had and still have a ball with that gun, unfortunatley it can't take the hotter loads or slugs, so I have to shoot vintage shells in it. Recoil is almost nothing, but still a highly recommended way to "start" into a double barrel two trigger side by side gun.

Then I bought a nice Italian double barrel slug gun (for quite a bit more $) but it came with one trigger. I had to retrain myself going from a double to a single trigger, but it did not take long. The holding of the gun, mounting & swinging a double sxs, the wrapping of the finger, etc. etc., were all the same.

I was thinking about your current need to get some "training" and one approach might be to buy a double barrel coach gun or a 24 inch older double shotgun. You can get these pretty cheap, ie, like I did, around $200.
Start out with light loads and work your way up and since you have plenty of RECOIL experience it is not like you are a NEWBY. So, you won't need many light loads. But hey every first few shots (in my opinion) are best with lower powder loads just to get the feel.

But I think the $100 - $200 investment would give you more confidence and comfort heading off to hunt with a TYPE of gun you lack acquaintence with. Howcan you put a price on confidence and comfort zone feelings heading off to shoot big game? Plus, you can then have this little double barrel 12 gauge for around the house or for who knows what, you may really like it and use yours... that is what happened to me... that was maybe one of the best $180 bucks I spent.