In her handspinning video, Mabel Ross took two rolags of wool from her hand-cards, laid one on top of the other and co-drafted them into a longer piece of the same diameter of the initial rolag. This would be one way to combine fibers without handcards as well, working from roving or top of the fibers.
Something like this would work with merino and silk. They may draft a bit differently, but with some practice it should even out.
Another thing to do would be to hold them together and draft them as you spin. Then, you can choose how much of each to draft as you go, varying the ratio of Merino to silk for more variability in the yarn. It can be interesting to have spots that are just Merino, just silk, and more of one than the other, as well as more even spots.
(Editor's note ... in case you are wondering ... I am not directly quoting the questions, but am writing them in my own words after re-reading my answer. And I just may re-edit my answer to better explain myself, too (grin).)
...And I notice that I just can't wait to post ... this was my very most recent that I can find posting on Spindlers, edited a bit. Just haven't gotten through the recent postings there to have a newer one to post. Tomorrow. Now, it is time for the evening rooibos (African red-bush tea).