使用正则表达式：
前任：

import re

# file_one.read()

one = """date : 26/04
test_name : Cs
link1 : xyz
link2 : abc"""

# file_second.read()

second = """Pending of taking test {test_name} was due on {date}. Follow the below link
X_link : {link1} --> replace with xyz from one.txt
y_link : {link2} --> replace with abc from one.txt"""

for k, v in re.findall(r"(\w+)\s*:\s*(\w+)", one):
    second = re.sub(f"{{{k}}}", v, second)    # substitute value
print(second)

输出：

Pending of taking test Cs was due on 26. Follow the below link
X_link : xyz --> replace with xyz from one.txt
y_link : abc --> replace with abc from one.txt

你的第二份文件已经准备好了 format 支持的占位符。所以配方如下：
将第一个文件解析为替换字典。
逐行读取第二个文件。
对于每一行，根据dict替换内容。
打印格式化行：

replacements = {}

with open('one.txt') as file1:
    for line in file1:
        name, value = line.split(':')
        replacements[name.strip()] = value.strip()

with open('secon.txt') as file2:
    for line in file2:
        print(line.format(**replacements), end='')

这样就可以解析文件。相对简单

def parse(text):
    lines = text.split("\n")
    data = {}
    for line in lines:
        key, value = line.split(":")
        data[key] = value
    return data

（我假设读取文件不是问题）
如果使用jinja，替换文件内容会更简单。然后你可以这样做：

template.render(test_name=data["test_name"])