The app is tied to the purchasing account. The best way to get the app to other devices with the same operating system is by Family Sharing: https://support.apple.com/en-us/HT201088 (works only for iOS 8 and higher).
Also, you can sign in with your first account and install the app, according to the following tutorial:
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Sign off from iTunes on your second device
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Sign there with your first account
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Find the app in your Purchased apps and install it
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Sign off your first account (app license data stay stored in the iTunes cache)
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Sign back in with your second account