Hi Ragtaggle. Here's some ideas to help, not all messy, but in my experience, very worthwhile as they're open-ended enough to allow two different ages to play. They probably won't play much 'with' each other (mine didn't at that age, anyway) but hopefully will be interested in their own thing enough to give you a lovely peaceful time! Anyway, first a handy tip;
A folding wallpaper paste table, (about £8 from our local DIY store) legs cut down to toddler height, makes a great space for them to move around and saves chairs & laps from falling debris when doing messy things. Even better in the garden!!! It can be covered with one sweep of old wallpaper, masking taped to prevent slipping (no newspaper needed), this can then be either covered with subsequent layers as needed, or just removed in one roll to your recycling bin!
Generous splodges of paint dolloped onto table top by adult can be explored with fingers, corks, (so long as 10 monther doesn't eat them)brushes, combs, cotton wall balls...(I've found a choice of two things at a time means they really explore, too many is too daunting)
Paint splodges can be patted with semi-inflated balloons, driven through with toy cars, or add cooked cooled spaghetti for some squishy moments.
Pieces of guttering and a selection of small cars or balls (again, depends on 10monthers eating habits!!) are good; gutters can be propped to make a slope. It is amazing how endlessly fascinating this seems to be, and how quickly the older ones try to put pieces of guttering end to end to make longer travelling times possible! (If indoors, recommend rolled up towels along the bottom of sofas, tv cabinet etc to save endless retrieval of rolling stock!)
Baby baths,(very cheap second hand)pieces of clear tubing (available from fish shops at about a £1 a metre), funnels, small empty bottles, jugs, water coloured with food colouring. Cover floor with towels first if you have to do it indoors, but best outside.
Minds gone blank now, if I think of anything else I'll let you know!