It is possible to have both although it is not very common, from what I have read. It might be necessary to see an MS specialist and a lupus specialist to distinguish - not all neurologists or rheumatologists specialise. There are a number of other conditions that can be lupus related that can have MS type symptoms.
What tests are you referring to ?
One condition that can cause MS like symptoms is a tendency -to- blood clotting condition called antiphosphlipid syndrome, APS. The tests for that are called ' lupus anticoagulant', a series of clotting tests, and for anti cardiolipins. They are often part of routine lupus work up these days.
I'm currently undiagnosed but still being investigated for both MS type neurological symptoms and lupus. I've had major symptoms for nearly three years now, with (in hindsight) many smaller relapses/flares for up to 20 years before that. I have seen neurologists who haven't found anything conclusive, and a rheumatologist, who considered lupus but didn't start treatment because my ANA was only 1:40. I am about to go off to yet another neurologist, on the advice of my GP, who still wants to get to the bottom of my progressive disability, and also to be reviewed by the same rheumatologist I last saw 18 months ago. I have heard of people who have MS and lupus, but I have also heard of people first diagnosed with MS and then it changed to lupus, and I've heard of people with lupus who had symptoms strongly suggestive of MS, but didn't have the MRI evidence to back that up. In short, I think the neurological symptoms do overlap quite considerably, and even some of the muscular symptoms can get confused. I know I couldn't tell you if my chest tightness was costochondritis or pleuritis (from lupus) or the so-called MS hug. Also, is my back muscle pain due to MS muscle spasm or generalised lupus ache? I'm just continuing to see both Neuro and Rheumia until one or the other decides they have enough evidence to treat me for something. I hope you get your answers soon.