To Pula, a nice and scenic way is to go via Milano - Trieste - Divaca - Buzet. But you'd need a night's rest somewhere.

To Split, it's possible to go that way to Divaca and then Ljubljana to Zagreb to Split.

Or back in Switzerland to Buchs (possible to go another route on the way back) and then by night train to Zagreb and from there to Split.

Or you may head to Ancona in Italy and go by ferry to Split. 

Depending on the date, it's also possible to do ferry hopping from Pula to Zadar and go by bus to Split from there.

There are many options... 

Thank you - I have tried the NightJet website and there doesn’t seem to be any avaiability whatsoever in any category or maybe I’m reading it wrong

Depends on the date you are looking for...

i’m looking at Mid June 16th onwards

If you are thinking of the night train from Zürich via Buchs to Zagreb, there's a lot of availabilty in all categories for the dates I tried. 

But that's a croatian EuroNight, no Nightjet.

Would you be happy to post a link to this booking site please?

tickets.oebb.at

Do NOT use "Seat reservations only” but add the Interrail/Eurail discount and select "One-way tickets and day tickets”. On 16 and 17 June there's availability in all categories.

If I book for 2 people in the sleeper will there just be us in the compartment?  showing as 113.40 euros

The price doesn’t match. 113,40 € should be the sleeper supplement without the base ticket for just one person. So I think it’s not what you’d like to book. 

Two persons in a private cabin should be 139,80 € all together (assuming you are booking pass holder reservations with ÖBB on the previous given date).

So you should better verify your booking or provide the specific date here or post a screenshot. 

i have put in the 18th June and it comes to 139,80 euro as you say  Thank you very much for your help I have booked that now