SPI's Universalis had a similar issue, where it wanted to know the environmental modifier if you were in your native environment or outside it.
Here is the page with the chart they made for it.
So what could be done is a similar chart where players mark what sorts of environments they have had training in, and each 'step' farther away from that training they make it 1 pt harder for target numbers. Adding in another training creates a new 'peak' in the chart, and numbers are adjusted accordingly.
For example, using the 'Mountains' row in the chart, the PC has received training in 'Cratered' type terrain. Their part of the chart would read: 3_4_3_2_1_0_-1. The partner selects partial training in Forest Mountains, and their chart looks like -3_-2_-1_0_1_2_1
Later on, they cross-train, to bring each other up to speed. By the power of their charts combined, it gets redone as follows:
#_4_#_#_#_2_# (their respective trainings are put in)
3_4_3_#_#_2_# (neighbors of highest are higher than others)
3_4_3_2_#_2_# (neighbor is reduced by 1 again, and matches another number)
3_4_3_2_1_2_1 (neighbors are reduced by 1 again, and the Survival chart is complete for both characters)
Obviously if we could use the original 2-D chart it would be much easier compared to using multiple 1-D charts
It also opens itself up for exploitation where someone will choose to avoid extreme training since it is better to be as close to the middle as possible:
(4_3_2_1_0_-1_-2 vs 1_2_3_4_3_2_1)
Others might figure that since living in the central areas is inherently easier and there will likely be locals, some extreme training might be useful.