Afaik there is no Austrian or German SIM card that would allow data roaming in the other country at reasonable prices, at least not if you need more than a few dozen MB of data volume. This leaves separate SIM cards from each country as the only affordable option. For Internet access in Germany the good old advice is still Lidl, about which you'll find all necessary information at
LIDL mobile - Prepaid Wireless Internet Access besides of the several hundred threads about Lidl on this board.