  We demonstrate on-chip coherent beam combination of two waveguide amplifiers
on Er$^{3+}$-doped thin film lithium niobate (Er: TFLN) platform. Our device is
built based on an electro-optic modulator fabricated on Er: TFLN. The output
power of the coherently combined amplifiers is measured as high as 12.9 mW,
surpassing that of previous single waveguide amplifiers based on
Er$^{3+}$-doped thin film lithium niobate platform.
