Paladin 3 (Oath of Conquest) / Cleric 1 (Forge Domain)
Source: Xanathar's Guide to Everything.
How are character categories interpreted correctly? As a paladin, has he sworn an oath of Tempus and at the same time prays to Gond? Is that possible or is the explanation much easier?
In the rules there is no limit in this regard, the question arises in knowing especially how to interpret or focus correctly on the character.
Another dubious example of multiclass: Cleric of the domain of trickery (Loki) and Druid. If he were a cleric in the domain of nature and at the same time a druid, it would be easy to understand or at least assimilate, but the Loki / Druid mix did not fully understand how he focuses to play it correctly.
Another simple example taken from the player's manual: cleric of the domain of war and druid. What god does he pray? Or is it really supposed to pray to several and there is no problem in a polytheistic environment?
I have put several examples not because of the idea of needing an answer for each of them, but rather I looked for a generic answer for any mix that could be made with a multiclass character.
Firstly, Paladins do not need to worship a god at all, since "Although many paladins are devoted to gods of good, a paladin's power comes as much from a commitment to justice as it does from a god." (PHB, pg. 82)
To actually answer your question, though, yes a PC may worship several gods if they wish. This is outlined in the PHB, pg 293:
From among the gods available, you can choose a single deity for your character to serve, worship, or pay lip service to. Or you can pick a few that your character prays to most often.
It makes sense from a narrative perspective not to worship multiple gods (especially not those you are trying to get powers from, i.e. as a Cleric) that clash with one another, but otherwise it's perfectly reasonable to pray to different gods in different environments or situations as makes sense.
The pantheon section in the PHB actually explains how many people worship multiple gods depending upon the situation. I think the examples are more mundane (IE: Pray for sun to a sun god, calm seas to a sea god), but the basic principle still applies.
There is no real conflict here, and the RP element is that each class is weaker than a full class because of the slightly lesser connection to each god.
The generic answer is that it depends on the setting. The background of the setting (either specific or implied in the case of the core books) had details one need to decide whether the blending make sense or not. Then talk with the player about how they are going to handle the combination.
Throughout human history many cultures have people venerating different spiritual entities. Yet there are some combinations that are rare due to conflicting theologies or customs. The religious history of the subcontinent of India has numerous examples of different religions intersecting.
A specific source to read on how this works is to read up on Syncretism. A cultural process where different belief systems or schools of thought are combined.
In this topic referring to having a character whose categories serve different deities, we must distinguish two cases on a global level: without a concrete game world and with a concrete world, like the Forgotten Realms. Although the rules apply equally in both cases, nuances in dealing with specific gods (Tempus) and not with a simple domain of an unnamed deity count. The idea is very simple: the gods fight each other. In what position would a character be who worshiped opposing deities? Unthinkable.
This is perfectly described by Ed Greenwood himself: Could a paladin of Tempus serve a an additional god?
At this point, my conclusion is that if the developing party has no concrete world or background of the gods then everything is acceptable; but if it has a background (Forgotten Realms, Dragonlance ...) then only one god is worshiped per character. That does not mean that he reverences and respects the rest of the deities that coexist in a pantheon, but when dedicating and consecrating the adoration of a character, only one deity should be done exclusively.