Let's say there are two PCs, both with 15 Strength. We're using the Variant: Encumbrance rules and there is a body that they want to try carry between them which weights 250 lb.
How would I calculate their combined lift? Would I just add their strength together? In that case they would be heavily encumbered which would slow their movement by 20 ft.
If you are using the Variant Encumberance rules (PHB 176):
If you carry weight in excess of 5 times your Strength score, you are encumbered, which means your speed drops by 10 feet.
If you carry weight in excess of 10 times your Strength score, up to your maximum carrying capacity, you are instead heavily encumbered, which means your speed drops by 20 feet and you have disadvantage on ability checks, attack rolls, and saving throws that use Strength, Dexterity, or Constitution.
A character with 15 strength can carry up to 75 pounds without being encumbered. Then up to 150 pounds with a movement speed reduced by 10, only when carrying more than that is the character heavily encumbered.
The rules do not specify how carrying an object together works. Ask your DM how they handle it.
The closest thing that we have is how carrying for animals that pull a vehicle works (PHB 155):
An animal pulling a carriage, cart, chariot, sled, or wagon can move weight [...] If multiple animals pull the same vehicle, they can add their carrying capacity together.
If you apply the same to two characters with 15 strength, then they can carry a total of up to 150 pounds without facing encumbrance penalties and up to 300 pounds without facing heavy encumbrance penalties.
So this depends on their gear: if they use gear that weighs less than 50 pounds combined, then they would be encumbered, but only when exceeding the total threshold of 300 pounds would they be heavily encumbered.
The advantage on the next ability check granted by the help action (PHB 192):
You can lend your aid to another creature in the completion of a task. When you take the Help action, the creature you aid gains advantage on the next ability check it makes to perform the task you are helping with, provided that it makes the check before the start of your next turn.
does not impact carrying as it doesn't take an ability check. Unless your DM says that it does - in that case it is still up to the DM on how that works out when the person that already helps you to carry also tries to give you advantage: ask your DM.
This answer ignores any equipment the characters are carrying i.e. their starting encumbrance is zero.
A character with a Strength score of 15 can carry a total of 225 (15 x 15) but if they carry more than 5 times their Strength score they are encumbered and their speed drops by 10 feet. More than 10 times their Strength score and they are heavily encumbered and move 20 feet slower.
Two characters carrying a 250lb weight will shoulder half of that weight each (this is not actually specified in the rules to the best of my knowledge and may therefore be subject to a DM ruling) but that means each character is carrying 125lbs. Easily under their maximum limit but over the encumbrance of total of 75 (15 x 5). Note that this is under the heavy encumbrance limit of 150 (15 x 10).
Even if you went the other way and combined their Strength scores to get a total of 30 that still doesn't reach being heavily encumbered as 250lb is less than 300 (30 x 10), which makes perfect sense, math is like that.
