Sometimes if you get yourself a more detailed map or, better, a trekking map (like Kompass) it will happen you'll find out that there's a path joining the two valleys that your first map showed unconnected: sometimes this path is a non paved route, 2 or 3 mt. wide, that is suitable for MTB (and only for MTB).
So riding along the westside of Lake of Como, when in Dongo (about 50 km. from Como) turn left up to Garzeno: at first you pant a bit because of long straight stretches a bit steep. A set of switchbacks allows you to rest and you can see how rewarding is the surrounding landscape: in front of you Monte Legnone the highest mountain in the region's lake and the Piona Cape, northward you see the end of Lake that is when Adda river enters the lake and, below you a large grass area. No more difficult till to Garzeno: here you have to take directions to Brenzeglio. The route become narrower and a bit ondulated: then after 4 steep switchbacks the routes go on unpaved: note a drinking fountain at your right near the white church). (at this point you're about 800 mt up to the sea level).
Now you're in Val Dongana. The unpaved route at first is not steep and you'll find also some descent stretch that gets you to Brenzeglio: a set of little houses with no electricity but still populated. A right bend to start a difficult stretch almost paved: from here the route becomes steeper and harder to ride but always wide. An endless set of switchbacks allows you to gain altitude: if you look behind you you'll see again on distance Piona Cape while in front of you the famous mountain Bregagno , Tabor and Gino. Up to you, you clearly see the amounts of stones building the next switchbacks you have to ride: the route becomes better and some stretch is made up of grass. You won't meet any flat stretch till Dosso di Brento (1450 mt) where switchbacks finish: a wide grass (if cows are absent) invites you to rest.
From here you see a white building: it's Hospiz il Giovo (1706 mt.) that is next flat stretch on your way. To get there the route coasts around the mountains at your right in continous and sometime steep ascent. At Hospiz Val Dongana and Valle S. Jorio are gathering. The Valle S. Jorio's route start from Garzeno as well by following brown road sign saying "Passo S. Jorio": I've never been to there but as far as I know it's well paved till 1350 mt. up to the sea-level then till to Hospiz Il Giovo it's a trekking path quite narrow (you're likely not to be able to go on bike here).
From the Hospiz you can see the remains of the path till Passo S.Jorio. Today this stretch is fully covered of snow (more than 50 cm.), so I decide to follow an alternative way (only for trekking people) till passo Traversa: from here you see a lot of things including Lago Maggiore, Monte Rosa and something looking like Mattherhorn (there's a lot of clouds) but, above all, the route that from Bellinzona - Carena takes (unpaved and only for mtb) to Passo S. Jorio.