Strawberry-Rhubarb is a great combination. Rhubarb has its own flavor, which is very nice and cam be exceedingly smooth when aged a year. But rhubarb has the uncanny ability to take on the flavor of anything blended with it, neither competing with the blend nor dominating it the way some other wines will. Proportions of each is really a personal choice, but I would mix up some trial blends and choose the best among them. I would tend to go strong on the strawberry just to ensure there is an adequate strawberry flavor to the wine, but you can then make test batches that back off from that strength. To do this, you should have one or more small beakers graduated in a milliliter scale. Mix 50 ml of each wine in a glass and allow to stand a half-hour before tasting. Mix additional samples using 40-60 strawberry-rhubarb and 30-70 strawberry rhubarb and taste all three blends (including the 50-50) in succession. To be fair, you should also try 60-40 and 70-30 blends, with the strawberry being the major component. Once you find the proportion that tastes best to you, then go ahead and blend larger amounts in this proportion.
Strawberry is of low-to-medium acidity and rhubard is of fairly high acidity, so the two ferment well together. The recipe below takes advantage of this fact but includes some white grape juice concentrate for added body. Any white grape juice concentrate will do, but the recipe specifies one you can buy in almost any American supermarket.
Trim all leaves from rhubarb. Do not peel, but wipe clean and cut into thin, quarter-inch lengths. Lay slices in bottom of primary and sprinkle sugar evenly over them. Cover with clean cloth and allow to sit 24 hours. The juice from the rhubarb will have largely turned the sugar to a syrup. Using a sanitized spoon or spatula, scoop the rhubarb slices into a nylon straining bag containing the strawberries (sliced if fresh, thawed and chopped if frozen). Tie closed and lay in primary. Stir in all remaining ingredients except pectic enzyme and yeast. Cover primary for 12 hours, then add pectic enzyme and stir. Recover the primary and allow to sit another 12 hours. Stir must again to ensure all sugar is dissolved and add activated yeast. Recover primary and set aside. Punch down cap twice daily for 5-7 days. Remove bag and allow to drip-drain (do not squeeze) for at least 30 minutes. Combine drippings with liquid in primary and transfer to secondary, topping up if required. When fermentation in secondary stops (3-8 weeks), rack, top up and refit airlock. Rack again every 6 weeks until wine is clear and no longer dropping sediment. Stabilize and sweeten to taste if desired. If no renewed fermentation in 30 days, bottle the wine. Age 3-6 months, but no longer than one year. [Author's own recipe]
My thanks to Mark for the request.