I hae done a LOT of solo live training-not a problem and definately focuses your mind.
I've done a reasonable amount of live partner shortblade training, with only one through-and-through arm injury as payment...longblade live training has two possible fatal flaws: one, if you or your partner get distracted or tired, someone dies, or is ruined for life (especially in a preindustrial level of medical tech), and if those veeerrry expensive swords happen to connect with each other hard, somebody's out a lot of $$$$$$. Edge to edge or edge to spine hard contact is usually the end of a Japanese sword unless it's awfully good. I have an old chisa katana from the 1600's with a parrying nick in the spine-and it's been polished back so many times that the hamon is only 1-3mm wide in spots, which tells me it got badly chipped quite regularly (the only reason I could afford it, even back in '88). Bokken are an excellent solution-still lethal enough to focus the mind, not likely to cripple at an accidental touch, and relatively cheap.( for example, a Viking sword cost the modern equivalent to a brand new Toyota pickup)
Really enjoying this thread