Solo, you are speaking of Basic Fantasy, but he's asking on the Iron Falcon part of the forum...
The character should have 2 xp pools of 5000xp each and 2 hit point pools, one with 3d8 (fighter) and one with 3d6 (cleric), and should be using the higher amount rolled.
The pc should gain xp equally divided in its both xp pools, level each class accordingly and using the higher rolled hp pool
If a class has reached its level limit it will not gain any new benefit, but the xp is still divided between them.
At 6,400 on both XP pools (12,800 total) the character would be a Fighter 3/Cleric 4, you would roll a d6 and add the result (+CON) to the cleric's HP pool, and use that if its higher than his current HP.
At 8,000 xp (16,000 total) the character will become a Fighter 4/Cleric 4, you then roll a d8 and do as above.
So... I made a table comparing the progression between Fighter, Cleric and Fighter/Cleric as way of knowing how viable is the multiclass.
- Comparisson.png (47.11 KiB) Viewed 5117 times
I color-coded it to know which one gets the upper or lower hand in level and combat, HP can be easily derived from levels, same as turning undeads and clerical spells.
Also, note that a Cleric/Fighter gets no better than a Cleric in their weapon and armor access.