If you ever see (ss) next to a team's name on a schedule and wonder what that means, the "ss" in parenthesis stands for split squad.
When teams split their squads, it enables them to play two games in one day (usually one at home and one on the road).
An example from the 2020 schedule
On that day, the Dodgers are playing two games, thus the split-squad (ss) designation next to their name.
So split-squads are the way a team can play a doubleheader in spring training, albeit of the atypical sort as there is no overlap of personnel (players or coaches) on either squad. That means no player plays in both games.
Aside from spring training, the sight of (ss) is absent on a baseball schedule at any level, but split squads are an indispensable feature of major league baseball's exhibition lineup of Cactus and Grapefruit league games, and their inclusion is certainly helpful to the schedule makers since there are an uneven amount of teams training in Arizona and Florida (15 each).
Every team usually has at least one split-squad date during spring training, and many teams have multiple days in which they play multiple games. That's easily possible in spring training since there are so many players in camp.
Besides evening out the schedule, the other benefit of scheduling split squads has to do with there being so many players in camp. While a team will have only a 25-man roster come Opening Day, roster sizes are much greater than that in spring training and split-squad games are useful for allowing many of the "extra" players more, or even some, playing time plus the corresponding opportunity for the coaching staff to evaluate their performance.
Who plays where?
That's particularly true in Florida, where driving distances to away games are much greater than they are in Arizona. So there's a really good chance you're going to see the equivalent of the "B" team at a road stadium in the Grapefruit League.
For example, look at the box scores from March 18, 2016 to see how the Red Sox split their squads to play a game at home in Fort Myers [box score] and away at Port Charlotte [box score]. The ballparks are 50 miles apart, but if you wanted to see what most fans would consider the "real" Red Sox that day then you only wanted to be at JetBlue Park.
Still, in other situations there might be a name or two you recognize regardless as teams are supposed to bring some regulars to road games. That's mainly because MLB realizes that paying customers want to see at least some normal representation for the opposing team. However, that unofficial "rule" is usually skirted when a team also has a game at home on the same day.
Because of the typically short commutes in Arizona, a team's road squad for Cactus League contests won't necessarily be a "B" (for backup) version. Case in point, the Cubs on March 7, 2016 had solid regular (important) player representation for both their home game in Mesa [box score] and road game 9 miles away in Scottsdale [box score].
As for finding out in advance which players are playing where, that's something you won't know very far in advance. It used to be there was no way to know until you arrived at the ballpark, but nowadays teams usually post their starting line-ups on their official Twitter account a few hours before first pitch (and first pitches, in the case of split squads).
Finally, here are a few things to know about and note when a team is set to have its split squad date(s):
Two games, two places
There are limited occasions in which both of a team's split squads play on the road -- that happened four times in 2017. Even rarer, a team can play both of its (ss) games at home, although that necessitates there be a day game followed later by a night game. That happened once in 2018.
And since 14 teams share a stadium, there's also the possibility of a team playing both its split-squad games in the same venue and be the away team in one and the home team in the other game. That takes day/night games to happen and did indeed occur once in 2018 (the Rangers and Royals were the teams involved at Surprise Stadium).
Game times at the same time
There are a few exceptions annually in which a split-up team will play a day and night game -- that occurred precisely 9 times in 2017 -- but about 90% of all spring training games are afternoon affairs, so a double dosage of day games are the norm for split squads.
Special occasion split squads
One annual example comes to mind: the Cubs have long sent a group to Las Vegas to play a pair of games at Cashman Field for what is billed as "Big League Weekend," as the Cubs play a split squad from another MLB organization. Meanwhile, the teams' other squad plays back in Arizona.