In theory, I think it's a good idea for toddlers, so they can stand. But I don't have a toddler, so I'm just guessing here (kinda like I thought babies would go to sleep on their own when they got tired-ha!).
Worth it, not worth it?
Fwiw, they will be 14m (adjusted age) apart.

Re: Thoughts on the "sit n stand" stroller
BFP #1 9/7/10, EDD 5/14/11, Violet born 5/27/11.
BFP #2 4/9/12, EDD 12/16/12, M/C Rory 4/24/12.
BFP #3 10/6/12, EDD 6/16/12., Matilda born 6/17/13.
BFP #1 5/2004 Karina Frances born February 6, 2005 8 lb., 6 oz.
BFP #2 8/2010 Hadleigh Abigail born April 7, 2011 8 lb., 11 oz.
BFP #3 7/2011 EDD 3/27/12 Missed MC at 12w3d (Sara Grace)
BFP #4 12/13/11 EDD 8/19/12 Praying for this little baby!
Daughter #1 - February 12, 2010
natural m/c March 11, 2011 at 8 1/2 weeks
Daughter #2 - January 11, 2012
Ectopic pregnancy discovered November 6, 2012 at 6 weeks
Daughter #3 - January 19, 2014
Started our exploration into the world of international adoption June 2012. We have no idea what this is going to look like but we are excited to find out!
We just bought one before LO#2 comes and I love it, it reminds me of a transformer lol. Our kids are going to be 16 months apart and my DS hasnt started walking yet so I feel like we needed it. It was a fair investment but I am enjoying it and DD isnt even here yet. Here is a link to the one that we got
https://www.toysrus.com/product/index.jsp?productId=12535573
mine are 16 months apart and I couldn't imagine getting a sit n stand at that age or until recently really.
The problem is toddlers are impulsive and if they want to get out they just dash out. Now DD has the verbal skills and patience to say "I go out now" and I stop and she steps out but really you will need something so they can be strapped in.
17 months apart here.
In the beginning we did the single for the toddler and wore the baby a lot.
We had a true double but only used it a hand full of times. I bought it off craigslist and sold it after only a hand full of uses.
We LIVED in our sit and stand as they got older so I think the option of the sit and stand with a removable 2nd full seat is a brilliant choice as it's kinda the best of both worlds.
By the time my toddler was 2 he wanted out of the stroller a lot. I don't take him to the zoo and on walks to keep him locked down - I wanted him to explore and learn how to listen and mind. The sit and stand offers transport when the toddler is tired or in situations where safety is key (parking lots) without having to get them in and out all the time.
I think it depends a lot on your older tot's personality. Some are content to sit and watch the world go by. Others want to be on the move.
My recommendation is to research now but to wait to make any decisions until the baby is actually here. Your baby will change a lot between now and the time you actually need a stroller for 2!
Total score: 6 pregnancies, 5 losses, 2 amazing blessings that I'm thankful for every single day.
I borrowed one the other day to take my 2.5 year old and 4 month old (25 months apart) to the zoo and it was awful. The 2 year old kept dragging his feet and because the sit part wasn't comfortable he wouldn't stay in it. It was heavy and hard to push and I barely got them both across the park without a major meltdown. A double stroller worked perfectly the time before. Even with this age difference DS1 was just not ready for a sit and stand. It would be fine for short trips where DD1 could walk most of the time and hitch a ride if needed, but he is too young to walk long distances on his own, and when it's close to naptime and I need to get them a long distance he's not going to cooperate.
As for the carrier, DS1 either wants to be held or is getting in trouble so that I have to pick him up, so carrying DS2 has never worked for us, I need to be able to pick up DS1 if I have to.