… when I enter eighth notes for soprano and alto parts, with stems pointing the same direction, there is a visible beam for each part, leaving the appearance (with beams for each part) that they are 16th notes …

Instead of entering two voices with single 8ths, enter - in the same voice - two noteheads on the same stem.

Enter the unison and 2nd intervals as Layer 1 & 2. The other things that need to be on a single stem, enter in Layer 1 and hide rests in Layer 2.

Here's a simple example. The soprano quarter note is in layer one, while the alto eighths are in layer two. When you get to the second beat, since they are singing the same tempo, but on different notes, you put them on one stem. Enter the alto A, and then tap the 3 key to enter the soprano note a third above.

If you can post a screen shot of a couple of measures, we can probably advise you on the best way to do it.

Thank you all for the help.

I had thought in terms of each voice being in a separate layer, but it looks like for my needs, I should not think in this way.

Right. Hymnal notation follows different rules, than “standard” SATB.

Oddly, none of the (ancient?) hymnals I am familiar with from England or the US put S&A or T&B on one stem, although I think I have run across the practice in other books I ran into tangentially.

I guess that dates me; doesn't it?

Many examples:

http://openhymnal.org/OpenHymnal2014.06.pdf