Question about exact meaning of causative sentence

Question

I'm trying to read this manga (風光る) for practice and I came across this short sentence

今、沖田や藤堂に立合わせています。 (see image below)

In the English fan translation it is translated incorrectly to

Now they're getting presented to Okita and Toudou.

I think they confused 立ち合う with 立ち会う. Even if it was 立ち会う (to be present, to be witness to), it wouldn't make any sense (to me at least) to translate it like this. So that translation isn't very useful.

Anyway, the part with the に particle is what I'm confused about. The speaker (Hijikata) is either letting or making someone face off in a match (which we see after two pages), but I don't know if that someone is the new recruits or Toudou and Okita (and others). Is the に particle marking the person that someone is facing off against or is it marking the person who is actively facing off against someone else (the doer) here? I guess it's not really crucial to get the gist of the sentence, but I would still like to make sure what the exact meaning is.

I think it's probably the former, so something like:

Now I'm letting (the new recruits) face off against Toudou and Okita (and others).

The other option is:

Now I'm letting Toudou and Okita (and others) face off against (the new recruits).

Answer 1

In this case, "someone" is the four recruits. "Now I'm letting the new recruits face off against Toudou and Okita" is the correct interpretation.

立ち合う is an intransitive verb that does not take を when in plain active voice. 彼に立ち合う (or 彼と立ち合う) means "to face off against him" or "to play a game with him", but we do not say 彼を立ち合う whatsoever.

This means, when you build a causative sentence from this verb, the "causee" will be marked with を, just like you say 息子を学校に行かせる and such. In other words, 沖田や藤堂 cannot be the "causee". The sentence in question is actually "今 彼ら4人を沖田や藤堂に立ち合わせています", but 彼ら4人を was omitted because it was mentioned right before this sentence.

沖田や藤堂を立ち合わせています would have meant "I'm letting Toudou and Okita face off against the new recruits".

See: Causative Form - Difference between 子供に本を読ませる　and 子供を本を読ませる