The most negative reaction is always taken. If my character only joins House Redoran, members of the temple like him. If he then joins the fighters' guild, the boost in disposition from House Redoran disappears and drops below the standard disposition (e.g. 40 for 30 personality). Is this normal and either way, how can I change it?

The Fighter's Guild and the Temple don't really get along. A disposition decrease is normal in this case.

Yes, but should such a character's standing in House Redoran make up for that even if he'd be less popular with than someone who's only in House Redoran. Shouldn't the +2 Faction reaction drop to +1 rather than -1?

Why not just join the Temple as well? The disposition boost from joining a faction is almost always greater than negative modifiers from opposing factions. And you can be a member without actually having to do anything for them.

Gaining ranks exacerbates negative reputations. In general, be wary of rising too high in a native faction if you want to stay in the Empire's good books, and vice-versa.

Or you could just go all-out with the Telvanni, and be hated by everyone :angel:

In the mundane world do you always like everyone you know? Do they all like you?
Get used to it.

EDIT: I wrote the above last night when I was half asleep. Sorry for sounding catty.
The Temple and House Redoran are conservative Dunmer factions and tend to resent Imperial Guilds as occupiers.
If you do some business with Redoran and Temple Merchants and use a little flattery and Bribery in the course of the transactions it will help their dispositions.
Also if your Personality is only thirty use or train your various skills (Speechcraft, Mercantile, and Illusion.)to improve that attribute. It helps a lot interacting with NPCs. There are a few members of the temple (and House Redoran)that will only deal with faction members though no matter what.
Hope this is more helpful.
However if you're RPing and identify with your char, remember the first paragraph.

Back again.
It's been a while since I played House Redoran. I've never really noticed the differences in Disposition between factions because they seemed fairly inconsequential.
Today I was looking around UESP though. Acording to the WiKi House Redoran actually should have a positive disposition toward the Fighters Guild. The Temple has a negative disposition but as I say nothing of importance.