Having two instances of DS running at once, unless they are different versions (production and public beta, say) is very risky as DS will clear the temp folder, including optimised textures, on File>New or on launch so it is easy to lose the .tdl files and end up with a blank render. You may also have unexpected results when closing as DS saves its layout then, so if you changed anything and closed that version last it would be in effect at next launch but if you closed the unchanged version last the changes would be lost.