I was just coming on here to post some musings having had some great advice previously about doubles!
So long story short my eldest was 2 years 3 months when her sister arrived so a bit older than your LO will be but probably close enough for my experience to be vaguely useful.
I loved my single travel system and still do (maxicosi Andorra, love it to bits). In the end I picked up a secondhand Phil and Ted's Vibe which is the usual tandem for them. I wanted one I could put a car seat on and also use with newborn. Well in the end I never once put the car seat on it, nor did I use the baby and toddler setting as just didn't understand how it could work for everyone.
What happened was either the baby went in the sling and the toddler in (and out) of the single pushchair for example on a day out. OR the baby went in the single pushchair pram and the toddler walked, for example in the shops etc.
Now baby is 5 months and a bit heavy for the carrier for any length of time BUT the toddler walks even more than she did before. So my plan is that the single will still be the go to as can adjust for toddler and put baby in sling if it becomes necessary at the end of the day out.
So what did the double get used for? Only for walking in our village where the pavements are non existent in places and I wanted toddler and baby both securely fastened in. That's it. Plus it allowed me to leave the single in the car.
I managed to tip the double with just the baby in it the other day but I should ahe realised she was heavy enough for that to happen now. The basket is almost unusable in double mode. It weighs A TON, even with only 20kg of child in it (toddler very slender, baby less so) and I'm by no means fragile or unfit in any way. If you had any joint or fitness problems it would be horrendous.
So I'm about to donate my double to charity. I'm torn between getting a buggy board or maybe a cheap and light side by side but on reflection the double tandem wasn't the right choice for us at all. Luckily I paid £50 for it and I feel I've had enough use out of it to not be sorry to be giving it away.
Ironically it's actually pretty nice as a single so it might be that with your older child being a bit younger you might find you get plenty of use from the tandem as it is really simple to take the seat on and off, genuinely not a barrier at all. It's less way to slot the baby into it though... It also helps if you don't have a single that is almost perfect for what you want too, if only it had two seats 😂