To concatenate with BSD Join I want to concatenate in BSD Join. 

Data

Model Decreasing Constant Increasing
2025 75.5 82.5 89.5
2035 65.5 82.5 102
2050 49.5 82.5 112


and error configuration in ybar

Model Decreasing Constant Increasing
2025 2.5 2.5 2.5
2035 2.5 2.5 5
2050 7.5 7.5 7.5


which should be 

2025 75.5 82.5 89.5 2.5 2.5 2.5
...


Unix variant join A B does work according to my friend. 

Outputs from Head

$ head model1_ave.dat
Year Decreasing Constant Increasing
2025 75.5 82.5 89.5
2035 65.5 82.5 102
2050 49.5 82.5 112
$ head model1_ave_err.dat 
Model Decreasing Constant Increasing
2025 2.5 2.5 2.5
2035 2.5 2.5 5
2050 7.5 7.5 7.5


How can you concatenate the two files in OSX Join i.e BSD?
join works for me here:

pse@Mithos:~/t$ join -1 1 -2 1 t1 t2
Model Decreasing Constant Increasing Decreasing Constant Increasing
2025 75.5 82.5 89.5 2.5 2.5 2.5
2035 65.5 82.5 102 2.5 2.5 5
2050 49.5 82.5 112 7.5 7.5 7.5


In case there are non-matching header lines in each file you need to get rid of them first, otherwise the joining will fail.

pse@Mithos:~/t$ join -1 1 -2 1 &lt;(grep -v Year t1) &lt;(grep -v Model t2)
2025 75.5 82.5 89.5 2.5 2.5 2.5
2035 65.5 82.5 102 2.5 2.5 5
2050 49.5 82.5 112 7.5 7.5 7.5