SLG is the expected value of total bases ("TB") of an at-bat. You would use it slightly differently from batting average, since it's measuring a different thing.
Let's see some examples.
Player A has a .300 BA, .450 SLG, 1.5 SLG/BA ratio
Player B has a .280 BA, .320 SLG, 1.1 SLG/BA ratio
You can use batting average to measure, for example, the probability that one of two players gets a hit in their next at-bat. That would be
1-((1-.3)*(1-.28)) = 1-(.7*.72) = (.496). So these two players, on average, will get one hit between the two of them about half the time.
You cannot determine that with SLG, of course, as SLG does not measure proportion of at bats with hits. SLG measures total bases, though, so you can use that.
For example, in this case, you would say that the expected value of Player A's total bases after one at-bat is 0.450 (his SLG). So, in an average at-bat Player A records 0.45 bases. Player B, similarly, records 0.32 bases in an average at-bat. This all assumes that it is an offical at-bat, and not a walk or similar, of course.
If you want to combine the two, then, you would add the terms together. You would say, "The expected value of total bases after both Player A and Player B bat is 0.32+0.45 = 0.77", meaning after the two of them bat, on average 0.77 bases are recorded.
You could actually do this with batting averages, too, of course. 0.300 is also the expected value of the "hit" statistic after an at-bat. So after the two players both bat, the expected value of "hits" for the team for those two at-bats is 0.3+0.28 = 0.58.
This is fairly easy to see; you already calculated the joint probability of at least one hit (0.496), and the probability of both of them getting a hit is easy (0.3*0.28 = .084). Sum those two together and you get 0.58 again.
Another thing you can do with regards to conditional probability is combine batting average and slugging percentage. Batting average says "This is the probability of a hit", and Slugging percentage says "This is the expected number of bases from hits", right? So E(TB|H) is reasonable to calculate, and is simple (and this should be obvious): SLG/BA.
So 0.45 is SLG for player A, and 0.30 is BA for player A, so E(TB|H) = 1.5. Player B has a 1.1 SLG/BA ratio, so E(TB|H)=1.1.
What this means, though, is that you can use that figure to discuss the expectations between the two. If you want to know the expected number of bases given that one of them gets a hit, for example, you would say
E(TB|H) = Sum( (E(TB|H)*E(H) for each player) / (Joint probability of E(H))
= (E1(TB|H)*E1(H) + E2(TB|H)*E2(H)) / (1-(1-E1(H))*(1-E2(H)))
= (1.5*0.3 + 1.1*0.28)/(0.496)
That is, when at least one of them gets a hit, the expected value of total bases is 1.55 from the two of them.
SLG works well with other statistics, to some extent, this way; you can derive things like isolated power (ISO), which tells you what proportion of their slugging percentage comes from XBH explicitly. You can't explicitly calculate the probability of an XBH from it, though, since the proportion of 2B|3B|HR can vary with identical SLG/AVGs. (Take one HR and one Single, and make them 1 2B and 1 3B, for example; identical SLG and AVG for those two results.)