IMO - Sit paddling is a great cross training method but I do find it more strenuous on my back and in general.

My body always feels more battered after a few hours sitting, folded in half at the waist, single or double blade.

I'm a builder and have been fighting back issues most of my adult life.
Tried, but could never really get long into kayaking since I always found the positioning rather brutal. Instead, I found myself always going back to rowing - not skulls, but traditional hulls - like whale boats and dorys.
Started rowing when I was 5 when I was living and sailing aboard a 100' schooner and subsequently, depending on where i was living, have been at it ever since.
I currently live on the San Francisco bay and row a 13 1/2 foot Chamberlain Dory. It augments beautifully the workout I get from SUPing.
For what it's worth I don't use a sliding seat. Do't' have anything against it; it's just the traditionalist in me.
It's worth noting that when most folk think of rowing they think of rowing backwards - which of course is typical. However, I mix it up all the time with turning and facing the bow thereby facing forwards - what I call
pushing.
Additionally I've an oarlock fastened to the transom so I also stand up and skull with one, sometimes two, hands.
Cant' recommend it enough.

Lastly, I hope it's ok to introduce, in this thread, my thoughts re: an alternative water activity somewhat similar to kayaking given the OP's concerns. Just wanted to help out in case he might find this a good option.

Yo, take this over to sitdownzone.com, kooks. You're hogging all the threads.

I've been paddling surfski and SUP for years.  I came to SUP from Surfski, and still paddle surfski.  I probably paddle distance sup 2/3 and surfski 1/3.  Both are fun. 

I can't comment on how they complement each other as physiologic cross training.  They do use most of the same muscles, and a few different muscles.  However, I have to disagree with skibike; if you don't think surfski is a whole body workout, you're not getting enough leg drive. 

It's great having an alternative paddle sport for windy days.  Surfski is more efficient (faster), so you can paddle more effectively iagainst the wind, and against strong currents.

Downwinding in a surfski is a blast.

Until recently surfski had a steeper learning curve,  but now there are entry level ski options that make it easy to get started. 

Surfskis and SUP are great complements to one another.
The more I am on my surfski, the more I improve on my SUP - and vice versa.
It's just amazing actually.

They both have advantages, but I must admit that ocean paddling can be done more often on a surfski than on a SUP. Surfskis are less affected by wind-moods, so I am "less picky" on the day's conditions.

They both use the entire body.
Using good technique on a surfski will have your lower body do a lot of work as well.
On a SUP, however, I find myself using more mobility (ankles, for instance, move in all sorts of directions, whereas little movement of the ankle is present on a surfski).

Back problems can be common - on both.
On a SUP, because you break at the waist when paddling. So if you use a poor technique, you could hurt.
On a surfski, because of your seated position and failing to keeping your back straight while paddling.
You should not have a back problem with good technique - on both.

Back to square one: I feel great paddling both.