merge xray Dataset

Trying to merge / combine to xray.Datasets:

In [161]: o1 = xray.Dataset({'x':(('t'), (np.ones(10)))}, coords={'t':np.arange(10)})

In [162]: o2 = xray.Dataset({'x':(('t'), (np.ones(10) * 2))}, coords={'t':np.arange(5,15)})

In [163]: o1
Out[163]: 
<xray.Dataset>
Dimensions:  (t: 10)
Coordinates:
  * t        (t) int64 0 1 2 3 4 5 6 7 8 9
Data variables:
    x        (t) float64 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

In [164]: o2
Out[164]: 
<xray.Dataset>
Dimensions:  (t: 10)
Coordinates:
  * t        (t) int64 5 6 7 8 9 10 11 12 13 14
Data variables:
    x        (t) float64 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0

In [165]: o1.merge((o1 + o2), join='outer', overwrite_vars='x')
Out[165]: 
<xray.Dataset>
Dimensions:  (t: 10)
Coordinates:
  * t        (t) int64 0 1 2 3 4 5 6 7 8 9
Data variables:
    x        (t) float64 nan nan nan nan nan 3.0 3.0 3.0 3.0 3.0


This is what I want:

Out[AS I WANT]: 
<xray.Dataset>
Dimensions:  (t: 10)
Coordinates:
  * t        (t) int64 0 1 2 3 4 5 6 7 8 9
Data variables:
    x        (t) float64 1.0 1.0 1.0 1.0 1.0 3.0 3.0 3.0 3.0 3.0