Do you think the 583 guys will appreciate the more afluent members using their race as a practice session? Likewise anyone doing 620 for 'practice'.
I may be wrong, but I would have thought that 583 riders will be doing that series because they can't afford the £5000+ it takes for the 620 class.
Those who can afford it now have an advantage over those who cannot.
If all I could afford was the 583 series and was putting 100% into it, I'd be severely pi$$ed off if I knew half the 620 lads were using it for 'practice' for their race.
I personally think you should commit to ONE Desmo Due championship, and if you want extra practice - then gate crash another series, or do more trackdays - if you can afford a second bike.
Spirit of the series anyone?
I know this is a DSC race series, but let's not get carried away.
Pick your weapon and go for it.
I haven't decided what I'm doing yet (748 or 996 included!) but even if I could afford it, I would do one or the other and concentrate on that.