Splitting up the rosh chodesh reading when another follows it

Question

Shulchan Aruch, Orach Chayim 684:3 says (in my own translation):

If rosh chodesh Teves comes out on Shabas, we remove three Torah scrolls, and six people read in the weekly section and one in that of rosh chodesh, starting from "Uv'yom hashabas"; the maftir reads in the third scroll....

Mishna B'rura comments on "[six read] in the weekly section":

Likewise if seven, or more than seven, want to read, that's allowed also; the point is that there are no fewer than seven people between [the first two scrolls].

Shaar Hatziyun adds:

Likewise, five men can read from the weekly section

— meaning, and two from the second scroll. Presumably the same rules would apply to Shabas rosh chodesh Adar and Shabas rosh chodesh Nisan.

My questions are:

1. Does anyone know of a synagogue in which only five people read from the first scroll? Why do they do this?
2. There are some synagogues that add one aliya every week, so eight (plus the maftir) read. In weeks with three scrolls, do they add the aliya to the first scroll or the second?
3. There are some synagogues that add many aliyos every week, stopping as often as every three p'sukim (where allowed). Do they also split the second sefer between two people?

Answer 1

In response to #1 - I have never seen a Shul that gives only 5 Aliyos from the first Torah, and I would be surprised to hear that there is a Shul that does that.

In response to #2 - My Shul always gives an extra Aliya, and this past week they called up 7 in the first Torah, 1 for the second Torah, and 1 for Parshas HaChodesh.

In summation - The only Sefer which only one Aliya is allowable in is the Sefer Torah of the Maftir which is the third Sefer Torah. Although per the Sha'ar Ha'Tziyun you have mentioned the 7 Aliyos can be given from amongst the first two Torah's, to the best of my knowledge the common Minhag is to give only one Aliya from the second Sefer Torah.

Answer 2

Adding to Gershon's answer, there's probably a good reason as to why only 1 aliyah is given from the 2nd Torah. There are two rules regarding reading aliyot:

1 - At least 3 verses must be read

2 - At least 3 verses must be left to the end of the current parsha

There are two parshiot read from the 2nd (Shabbat / Rosh Hodesh reading). The Shabbat parsha has just two verses. The Rosh Hodesh parsha has five.

While, technically, one could read the Shabbat & 3 verses from the 2nd parsha for the 1st oleh and repeat verse 13 for the 2nd oleh, common (at least Ashkenazi) practice is to avoid repeating read verses for a "regular" (non-maftir) oleh. (Rosh Hodesh weekday is an exception, as there is no other way to break up the aliyot for 4 people.)

See this question for specifics about weekday Rosh Hodesh aliyot allocation.