I think FT92 has hit on the snag. Bias tires deflect differently than radial tires. What that means is when that trailer is loaded, the actual load being carried will be different.
If I remember correctly, bias tires would deflect more, which means the radial tires will be carrying more of the load. That's a problem, because when radial tires fail, the belts come loose and there is a lot of damage.
And contrary to your thinking, because of this deflection problem, it might be smarter to not pair on an axle, but to pair on a side.
And for reference: The reason why it was recommended not to mix bias and radials is that they react differently to steering input, so the ends of the car don't behave in unison.
And lastly, you should consider changing to 16" wheels. 16.5" tires are disappearing and will be getting more and more expensive as time goes on. It might actually be cheaper to do it now rather than later.