Ring sling sizing is quite flexible. One person can wear several sizes - all that will change is the length of the tail. For safety the tail of the ring sling should be no shorter than around 20cm (8") but apart from that the length of the tail is just a personal preference. Both medium and standard will fit most with larger sizes having a short tail with the size medium. You can wrap the tail around the rings a few times (or all the way around depending on fabric) to keep it out of the way or if the tail is too long for your likeing.
I found a great free pattern to add to my Links page. A tutorial about how to make your own babywearing jacket I'll add more patterns as I come across them. What are your favourite DIY links?
My husband was carrying our four your old when our toddler asked for a ride too. I tied the toddler on front in another mei tai. My husband said he could feel the weight but was not uncomfortable. The toddler did sag after a while I think because it was hard to tie the mei tai tightly enough, maybe because I had to cross it quite high on my husband's back. If both girls would like a ride at the same time in the future I think it would be better to tie my toddler on the front first and then the four year old could be tied on with rucksack straps. My daughters only want to be carried when they want to now so it's hard to experiment - so if you have any experience with tandem wearing I'd love to hear from you.
I love to sew. I have five curious and active kids who keep me busy!