Your question is not about writing to excel (this you know how to do) is about concatenating dataframes, you can use pandas.concat for that end.
In this example I create two sheets one with the datasets concatenated vertically, and the other with the datasets concatenated horizontally, as a bonus you know how to save multiple sheets to the same file.
cars = {'name': ['Audi','VW'],
'modell': ["A4","Golf"]
}
cars2 = {'name': ['BMW','MB'],
'modell': ["3er","e-class"]
}
## assuming that all your dataframes have the same set of columns
dataFrames = [pd.DataFrame(d) for d in [cars, cars2]]
with pd.ExcelWriter('pandas_simple.xlsx', engine='xlsxwriter') as writer:
vertical = pd.concat(dataFrames, axis=0) # if the dataframes have the same columns
horizontal = pd.concat(dataFrames, axis=1) # if the dataframes have the same indices
vertical.to_excel(writer, sheet_name='Vertically')
horizontal.to_excel(writer, sheet_name='Horizontally')