BERLIN (Reuters) - German ecommerce investor Rocket Internet and United Internet announced plans on Monday to each buy back shares held by the other company, cutting their cross shareholding.Rocket Internet said it would buy back up to 10% of its share capital in an offer starting on Dec. 10 and ending on Dec 18, with United Internet committing to accept an offer for its 7.4% stake.Berlin-based Rocket will offer 21.50 euros per share, valuing its buyback at up to 324 million euros (£278.34..