You also have to consider the density properties of the gasses. For example, hydrogen will rapidly rise whereas propane will settle to the lowever levels of the room/enclosure, so you could end up stratification of gas layers

So if these 2 gases were present this is likely to happen you would have different LELs at high and low levels. What is the ventilation in the room/enclosure - does it remove the gases quickly?

As regards which LEL to choose, more information is required. Are you relasing the same volume of each gas or greatly different gas quantities of each gas.

Hi JL

JohnW's solution would be a starting point assuming that the gases are relatively similar in nature.

It's all about stoichometrics and it's a very very long time since I did A Level Chemistry!!

The LEL will depend not only on the concentration of flammable gases but also what else is in the mix. 

So, you could calculate an LEL for the mix of flammables in typical air.

But that would differ, if you change the mix to reduce the level of oxygen (and any other oxidising material) e.g. by upping the level of inert gases such as nitrogen.

If in doubt, one for a specialist.

Like so many health and safety problems, sometimes there are easy decisions - "it's obviously a problem, and we can do X, Y and Z"; "the risk is negligible and here is the basis for that conclusion".

So, it's the ones in the middle where you need specialist input (including sometimes when there is obviously a problem but you have implemented the quick fixes).