Hi again, Gwendolyn.
Depends, if these are just sketches, that is, they are going to be extended and not only complemented with a third work, there should be no problem building tension as long as you know what prominence you want it to have. In the full orchestral piece you could alternate periods of tension with periods of relative "comfort" but increasing the tension in each "cycle". Something like this:
Some more details and opinions:
• The first ~15 seconds on the second part do a good job as an "opening" theme, they sound expansive to me.
• After listening a couple of times, I do not really think building tension too quickly be the problem if there's any. Do you want to build a theme based on a continuous flow of tension? Do you want rests, balance? Having the structure clear is key in this case (and very often I'd say, it always helps). Do you conceive this as a single movement work? In other words, no long rests and/or thematic splitting? Do you think there's something else that —apart from the general "mood" both pieces may transmit and the, likely, tonality sharing since both pieces go to F right after the beginning or in the very beginning— Is there any further specific connection between these works? Too many questions perhaps?
• Despite agreeing with ma buddy here @Henry Ng Tsz Kiu on the thicker texture of the Fantasy two, I consider the first one texture more elaborate and potentially better, as well as more emotionally charged. However, this is likely a consequence from my said judgement and preference of the 1st over the 2nd.
As always, thank you for sharing.
Kind regards,
Daniel–Ømicrón.