Although it wont be a huge factor, the writers would have to consider the international angle.

You cant link stories so heavily between series that they wont make sense to international audiences who may only get 1 of the shoes, or where one of the shows has a limited distribution in some countries.

Doctor Who is a great selling export for the BBC, Torchwood less so. Heavily linking the stories will make Doctor Who less attractive to the international market.

Not really, the main countries (Australia, the U.S. and Canada) have Torchwood and those that don't have Jack's explination of torchwood 3 in the sound of drums and the creation of torchwood in tooth and claw and the appearance of torchwood 1 (london) in series 2 to go by.

Maybe, but in the US at least Torchwood is on a much smaller network (BBC America) and as such is available to less people than the main show.

Theres also the matter of scheduling. While the BBC can schedule things here so that stories overlap and make sense they have no control in other countries, especially when different channels have the rights to the different shows.

It was the same here with Buffy/ Angel crossovers. 2 different networks had the rights and the scheduling was different.

Any crossover that the writers consider would have to be done in a way that makes sure the story makes sense even if you haven't or can't watch the other show.

Doing otherwise would risk damaging a successful export.

They probably won't it seems. Just found an interview with Julie Gardner from July:

iF: The end of series one of TORCHWOOD, the characters were on a bit of rocky ground emotionally. Where do things pick up with series two?

GARDNER: We’re hard into it. We’re a regenerated series and we hit the ground running. We’re not hanging onto any of that cross over angst where we were at the end of season one. We’re just moving forward.

(http://www.ifmagazine.com/feature.asp?article=2267)

Whether she means that torchwood series 2 will start long after Jack's return I don't know, but it seems that way.

But there again John Barrowman ruled out a crossover for series 1 lol.