Hi ,
Not sure about seriously valuable but suppose that depends on each ones level of spending ,low hundreds should get you a nice 18th c English example,the most frequently encountered double bowled glasses are commonly called double ended drams though I am unsure if there is any evidence to support this , maybe they were for doses of medicine who knows!!,these types are regarded as English and from around the middle of the 18th c ,personally I have never seen a 17th c one or one which may have come from Germany though that's not to say they dont exist ,they may well do.Another feature of these glasses is that most are deceptive to some degree,again the question arises , why ??,
Importantly though is how they were made,a genuine 18th c double bowled glass will have a pontil mark inside and at the base of one of the bowls,the only the way they can be made if both bowls are to have fire polished rims ,bowl one is blown as a bubble with or without the little stem part at the end,a second bubble is blow again with or without the little stem extruded,this second bubble is attached to the base of the first bubble,the first bubble is cracked of the blowing iron leaving the second bubble with stem and cracked off bubble at the other end, this cracked off end can then be shaped to the finished bowl and fire polished,a punty rod is then attached to the inside of the first bubble after fire polishing allowing the second bubble to be cracked off the other blowing iron this allows the whole thing then to be held by the punty rod inside the first bubble , the second bubble can then be finished to the required shape and size then fire polished,punty rod is then cracked off and hey presto a double ended glass .(sorry it's a bit long winded).
although I have never seen one i suppose it is possible to find a double bowled glass with flat ground rims,which I would assume to be continental and made differently to English ones
Anything that has 2 fire polished rims ,no pontil mark in either bowl,and with a hot weld in the stem I would say is not original.and more likely to be 2 broken glasses stuck together for some fun .
Attached are a couple of mine with the pontil mark inside at the base of the bowl .
cheers ,
Peter.