Sewing an extension onto synthetic sleeping bag?

https://www.aliexpress.com/w/wholesale-sleeping-bag.html?spm=2114.search0104.0.0.6f306032yhSgFb&site=glo&SortType=price_asc&g=y&SearchText=sleeping+bag&CatId=100005438&initiative_id=SB_20190712203901&needQuery=n&pvId=200001175-4100,380-200012008,200001580-200012000 There are a few cheaper options here (but also some definite non options) Stiching the second piece on should work if both bags are the same width at the join and you overlap by a couple of inches stitching both ends of the join. However it might not be comfortable and could ruin both bags. Its nearly always easier to cut something shorter than longer.

Geeves, you *do* know that you can shorten links to more manageable size? Eg - your link above is also : http://tiny.cc/wnjn9y Not difficult and only takes a few clicks - have a look at tiny.cc or bitly.com (and others)

Thanks @geeves I've already got a sacrificial donor bag. I'm especially looking for tips on how to sew the join so there aren't cold spots like a single seam which doesn't have insulation. Cheers. 😀

Synthetic sleeping bags usually have seams holding an insulation layer to each of the outer and inner fabric layers so that the inner and outer seams are offset from each other. So you'll need to cut both bags to allow for this, and hopefully the seam spacing for both bags is the same. Another option could be to send it to https://twinneedle.co.nz/ and ask them to do the modification for you.

I will have to remember to use link shorteners.

I recommend Twin Needle too. They do such a professional job. I got them to convert the bottom seam of my sleeping bag into a drawstring as I suffer very badly from hot feet and my bag was thus unusable in its former state. The drawstring is very robust and reinforced which is just as well as I apply a lot of stress to it in the small hours when it needs closing up again on frosty nights.