Anna, just came back as I have more time now to say that we went through the very same thing when Jasmine was born (mine were 13.5months apart).
I ended up doing a twisted version of wi/wo. Becuase I also didn't want the toddler's crying to wake the baby up, I would go in & stay with her until she would calm down & then leave (rather than the straight wi & wo). She would sometimes still cry when I'd leave & I would count till 5 or 10 before coming back & staying with her again... it took us a few weeks but she did go back to sleeping through. Another thing that was going on was the 2-1 transition (so thats why I asked when he is sleeping during the day). Bella was skipping naps & I thought it was because of the new baby but it had alot to do with her needing less daytime sleep & me needing to change her routine a little....
The baby was in our room for over 4 months so I am not sure if the toddler & baby are sharing rooms or not but for now I would have the baby with you until this is sorted out. We also had white noise in our room (fan) & still do... which is a great tool for blocking out the toddler screams during the day & at night.
Here's the link to sleep training methods for toddlers... have a look & let me know what you think
https://babywhispererforums.com/index.php?topic=63896.0Layla