How can I intensify the orange flavour in orange cake?
Citrus zest is where most citrus flavour in a sauce or baked good actually comes from because it remains in solid form, like herbs and spices. The juice adds some flavour but it turns into solution and gets spread out over a very large surface area/volume.

I don't know why you decided not to use the zest - are you using commercial orange juice? If so, that may be part of the problem too - processed juices are not going to be as flavourful as homemade, in part because of the mechanical separation of pulp (even if it is subsequently re-incorporated to make a juice "with pulp").

Orange juice is effectively your water in this recipe so doubling it means you need to double everything else. In other words you'll have exactly the same ratios and therefore exactly the same taste.

If it came out too dry then that means you either (a) baked too long, (b) overworked the batter, (c) didn't use enough oil or (d) didn't use enough sugar. Nothing to do with the OJ content.

If you really want to add more orange flavour beyond what the original recipe would give you, use orange extract, it's a highly concentrated orange flavour and is usually derived from (surprise!) the zest, or made to taste like one. But I would suggest to try following the recipe correctly before doing this.