While it's perfectly possible to ride a XT600 from Spain to Holland (plenty of us have done this), riding two up isn't something I would recommend you do on one.
The XT600 series of bikes are legendary Round the World tools BUT they do have their limitations in terms of comfort and two-up riding.
If you want a cheap reliable workhorse capable of effortlessly pulling rider and luggage around the world then the XT600 makes sense.
BUT A single cylinder is never going to be fun on high speed tarmac. Two up it's going to be even less fun.
If you intend to ride without a pillion and take B class roads and travel at no more than 60 MPH, then your idea becomes more worthy.
Not many Super Tenere's left in good / low milage condition. It's still very easy to find good low milage XT600E's in Europe. I'd be surprised if your Super Tenere hasn't already been ridden to almost destruction. If it's not been then it's a keeper. Bit like the excellent little DR350's - not many left. Pity.
I paid £1,600 for my mint low milage XT600E (1999). That's a lot of bike for little money. I'm sure it's possible to find good examples (Private sales) for between £1,000 and £1,500. Retailers will sell the same for a premium so my advice is to find a private sale.
Having said all that, some people do RTW trips on mopeds!!!!
I wish you well whatever you decide to do.
Triumph Bonneville 800 (2004), Yamaha XT600E (1999), Honda XBR500 (1986).